Communication terminal and base station

ABSTRACT

When being connected to a macro cell (S-MeNB) and a small cell (SeNB), a user equipment device performs a pre-handover process of disconnecting the connection with the SeNB before a handover process of switching a macro cell connected with the UE from the macro cell (S-MeNB) that is a moving source to a macro cell (T-MeNB) that is a moving destination along with moving of the UE, and after the handover process, performs a post-handover process of reestablishing the connection with the SeNB.

CROSS-REFERECE TO RELATED APPLICATIONS

This application is a continuation of and claims the benefit of priorityunder 35 U.S.C. § 120 from U.S. application Ser. No. 15/127,230 filedSep. 19, 2016, the entire contents of which are incorporated herein byreference. U.S. application Ser. No. 15/127,230 is a National Stage ofPCT/JP2015/057611 filed Mar. 16, 2015, which claims the benefit ofpriority under 35 U.S.C. § 119 from Japanese Application No. 2014-058327filed Mar. 20, 2014 and Japanese Application No. 2014-105050 filed May21, 2014.

TECHNICAL FIELD

The present invention relates to a communication system in which radiocommunication is performed between a communication terminal device and abase station device.

BACKGROUND ART

The 3rd generation partnership project (3GPP), the standard organizationregarding the mobile communication system, is studying new communicationsystems referred to as long term evolution (LTE) regarding radiosections and system architecture evolution (SAE) regarding the overallsystem configuration including a core network and a radio accessnetwork, which will be hereinafter collectively referred to as a networkas well (for example, see Non-Patent Documents 1 to 10). Thiscommunication system is also referred to as 3.9 generation (3.9 G)system.

As the access scheme of the LTE, orthogonal frequency divisionmultiplexing (OFDM) is used in a downlink direction and single carrierfrequency division multiple access (SC-FDMA) is used in an uplinkdirection. Further, differently from the wideband code division multipleaccess (W-CDMA), circuit switching is not provided but a packetcommunication system is only provided in the LTE.

The decisions by 3GPP regarding the frame configuration in the LTEsystem described in Non-Patent Document 1 (Chapter 5) will be describedwith reference to FIG. 1. FIG. 1 is a diagram illustrating theconfiguration of a radio frame used in the LTE communication system.With reference to FIG. 1, one radio frame is 10 ms. The radio frame isdivided into ten equally sized subframes. The subframe is divided intotwo equally sized slots. The first and sixth subframes contain adownlink synchronization signal (SS) per radio frame. Thesynchronization signals are classified into a primary synchronizationsignal (P-SS) and a secondary synchronization signal (S-SS).

Non-Patent Document 1 (Chapter 5) describes the decisions by 3GPPregarding the channel configuration in the LTE system. It is assumedthat the same channel configuration is used in a closed subscriber group(CSG) cell as that of a non-CSG cell.

A physical broadcast channel (PBCH) is a channel for downlinktransmission from a base station to a user equipment. A BCH transportblock is mapped to four subframes within a 40 ms interval. There is noexplicit signaling indicating 40 ms timing.

A physical control format indicator channel (PCFICH) is a channel fordownlink transmission from a base station to a user equipment. ThePCFICH notifies the number of orthogonal frequency division multiplexing(OFDM) symbols used for PDCCHs from the base station to the userequipment. The PCFICH is transmitted per subframe.

A physical downlink control channel (PDCCH) is a channel for downlinktransmission from a base station to a user equipment. The PDCCH notifiesof the resource allocation information for downlink shared channel(DL-SCH) being one of the transport channels described below, resourceallocation information for a paging channel (PCH) being one of thetransport channels described below, and hybrid automatic repeat request(HARQ) information related to DL-SCH. The PDCCH carries an uplinkscheduling grant. The PDCCH carries acknowledgement (Ack)/negativeacknowledgement (Nack) that is a response signal to uplink transmission.The PDCCH is referred to as an L1/L2 control signal as well.

A physical downlink shared channel (PDSCH) is a channel for downlinktransmission from a base station to a user equipment. A downlink sharedchannel (DL-SCH) that is a transport channel and a PCH that is atransport channel are mapped to the PDSCH.

A physical multicast channel (PMCH) is a channel for downlinktransmission from a base station to a user equipment. A multicastchannel (MCH) that is a transport channel is mapped to the PMCH.

A physical uplink control channel (PUCCH) is a channel for uplinktransmission from a user equipment to a base station. The PUCCH carriesAck/Nack that is a response signal to downlink transmission. The PUCCHcarries a channel quality indicator (CQI) report. The CQI is qualityinformation indicating the quality of received data or channel quality.In addition, the PUCCH carries a scheduling request (SR).

A physical uplink shared channel (PUSCH) is a channel for uplinktransmission from a user equipment to a base station. An uplink sharedchannel (UL-SCH) that is one of the transport channels is mapped to thePUSCH.

A physical hybrid ARQ indicator channel (PHICH) is a channel fordownlink transmission from a base station to a user equipment. The PHICHcarries Ack/Nack that is a response signal to uplink transmission. Aphysical random access channel (PRACH) is a channel for uplinktransmission from the user equipment to the base station. The PRACHcarries a random access preamble.

A downlink reference signal (RS) is a known symbol in the LTEcommunication system. The following five types of downlink referencesignals are defined: cell-specific reference signals (CRS), MBSFNreference signals, data demodulation reference signals (DM-RSs) beingUE-specific reference signals, positioning reference signals (PRSs), andchannel-state information reference signals (CST-RSs). The physicallayer measurement objects of a user equipment include reference signalreceived power (RSRP).

The transport channels described in Non-Patent Document 1 (Chapter 5)will be described. A broadcast channel (BCH) among the downlinktransport channels is broadcast to the entire coverage of a base station(cell). The BCH is mapped to the physical broadcast channel (PBCH).

Retransmission control according to a hybrid ARQ (HARQ) is applied to adownlink shared channel (DL-SCH). The DL-SCH can be broadcast to theentire coverage of the base station (cell). The DL-SCH supports dynamicor semi-static resource allocation. The semi-static resource allocationis also referred to as persistent scheduling. The DL-SCH supportsdiscontinuous reception (DRX) of a user equipment for enabling the userequipment to save power. The DL-SCH is mapped to the physical downlinkshared channel (PDSCH).

The paging channel (PCH) supports DRX of the user equipment for enablingthe user equipment to save power. The PCH is required to be broadcast tothe entire coverage of the base station (cell). The PCH is mapped tophysical resources such as the physical downlink shared channel (PDSCH)that can be used dynamically for traffic.

The multicast channel (MCH) is used for broadcast to the entire coverageof the base station (cell). The MCH supports SFN combining of multimediabroadcast multicast service (MBMS) services (MTCH and MCCH) inmulti-cell transmission. The MCH supports semi-static resourceallocation. The MCH is mapped to the PMCH.

Retransmission control according to a hybrid ARQ (HARQ) is applied to anuplink shared channel (UL-SCH) among the uplink transport channels. TheUL-SCH supports dynamic or semi-static resource allocation. The UL-SCHis mapped to the physical uplink shared channel (PUSCH).

A random access channel (RACH) is limited to control information. TheRACH involves a collision risk. The RACH is mapped to the physicalrandom access channel (PRACH).

The HARQ will be described. The HARQ is the technique for improving thecommunication quality of a channel by combination of automatic repeatrequest (ARQ) and error correction (forward error correction). The HARQis advantageous in that error correction functions effectively byretransmission even for a channel whose communication quality changes.In particular, it is also possible to achieve further qualityimprovement in retransmission through combination of the receptionresults of the first transmission and the reception results of theretransmission.

An example of the retransmission method will be described. If thereceiver fails to successfully decode the received data, in other words,if a cyclic redundancy check (CRC) error occurs (CRC=NG), the receivertransmits “Nack” to the transmitter. The transmitter that has received“Nack” retransmits the data. If the receiver successfully decodes thereceived data, in other words, if a CRC error does not occur (CRC=OK),the receiver transmits “AcK” to the transmitter. The transmitter thathas received “Ack” transmits the next data.

The logical channels described in Non-Patent Document 1 (Chapter 6) willbe described. A broadcast control channel (BCCH) is a downlink channelfor broadcast system control information. The BCCH that is a logicalchannel is mapped to the broadcast channel (BCH) or downlink sharedchannel (DL-SCH) that is a transport channel.

A paging control channel (PCCH) is a downlink channel for transmittingpaging information and system information change notifications. The PCCHis used when the network does not know the cell location of a userequipment. The PCCH that is a logical channel is mapped to the pagingchannel (PCH) that is a transport channel.

A common control channel (CCCH) is a channel for transmission controlinformation between user equipments and a base station. The CCCH is usedin the case where the user equipments have no RRC connection with thenetwork. In the downlink direction, the CCCH is mapped to the downlinkshared channel (DL-SCH) that is a transport channel. In the uplinkdirection, the CCCH is mapped to the uplink shared channel (UL-SCH) thatis a transport channel.

A multicast control channel (MCCH) is a downlink channel forpoint-to-multipoint transmission. The MCCH is used for transmission ofMBMS control information for one or several MTCHs from a network to auser equipment. The MCCH is used only by a user equipment duringreception of the MBMS. The MCCH is mapped to the multicast channel (MCH)that is a transport channel.

A dedicated control channel (DCCH) is a channel that transmits dedicatedcontrol information between a user equipment and a network on apoint-to-point basis. The DCCH is used when the user equipment has anRRC connection. The DCCH is mapped to the uplink shared channel (UL-SCH)in uplink and mapped to the downlink shared channel (DL-SCH) indownlink.

A dedicated traffic channel (DTCH) is a point-to-point communicationchannel for transmission of user information to a dedicated userequipment. The DTCH exists in uplink as well as downlink. The DTCH ismapped to the uplink shared channel (UL-SCH) in uplink and mapped to thedownlink shared channel (DL-SCH) in downlink.

A multicast traffic channel (MTCH) is a downlink channel for trafficdata transmission from a network to a user equipment. The MTCH is achannel used only by a user equipment during reception of the MBMS. TheMTCH is mapped to the multicast channel (MCH).

CGI represents a cell global identifier. ECGI represents an E-UTRAN cellglobal identifier. A closed subscriber group (CSG) cell is introduced inthe LTE, and the long term evolution advanced (LTE-A) and universalmobile telecommunication system (UMTS) described below.

The closed subscriber group (CSG) cell is a cell in which subscriberswho are allowed use are specified by an operator (hereinafter, alsoreferred to as a “cell for specific subscribers”). The specifiedsubscribers are allowed to access one or more cells of a public landmobile network (PLMN). One or more cells to which the specifiedsubscribers are allowed access are referred to as “CSG cell(s)”. Notethat access is limited in the PLMN.

The CSG cell is part of the PLMN that broadcasts a specific CSG identity(CSG ID; CSG-ID) and broadcasts “TRUE” in a CSG indication. Theauthorized members of the subscriber group who have registered inadvance access the CSG cells using the CSG-ID that is the accesspermission information.

The CSG-ID is broadcast by the CSG cell or cells. A plurality of CSG-IDsexist in the LTE communication system. The CSG-IDs are used by userequipments (UEs) for making access from CSG-related members easier.

The locations of user equipments are tracked based on an area composedof one or more cells. The locations are tracked for enabling trackingthe locations of user equipments and calling user equipments, in otherwords, incoming calling to user equipments even in an idle state. Anarea for tracking locations of user equipments is referred to as atracking area.

3GPP is studying base stations referred to as Home-NodeB (Home-NB; HNB)and Home-eNodeB (Home-eNB; HeNB). HNB/HeNB is a base station for, forexample, household, corporation, or commercial access service inUTRAN/E-UTRAN. Non-Patent Document 3 discloses three different modes ofthe access to the HeNB and HNB. Specifically, an open access mode, aclosed access mode, and a hybrid access mode are disclosed.

The individual modes have the following characteristics. In the openaccess mode, the HeNB and HNB are operated as a normal cell of a normaloperator. In the closed access mode, the HeNB and HNB are operated as aCSG cell. The CSG cell is a CSG cell where only CSG members are allowedaccess. In the hybrid access mode, the HeNB and HNB are operated as CSGcells where non-CSG members are allowed access at the same time. Inother words, a cell in the hybrid access mode (also referred to as ahybrid cell) is the cell that supports both of the open access mode andthe closed access mode.

In 3GPP, among all physical cell identities (PCIs) is a range of PCIsreserved by the network for use by CSG cells (see Chapter 10.5.1.1 ofNon-Patent Document 1). Division of the PCI range is also referred to asPCI split. The information about PCI split (also referred to as PCIsplit information) is broadcast in the system information from a basestation to user equipments being served thereby. Being served by a basestation means taking the base station as a serving cell.

Non-Patent Document 4 discloses the basic operation of a user equipmentusing PCI split. The user equipment that does not have the PCI splitinformation needs to perform cell search using all PCIs, for example,using all 504 codes. On the other hand, the user equipment that has thePCI split information is capable of performing cell search using the PCIsplit information.

Further, 3GPP is pursuing specifications standard of long term evolutionadvanced (LTE-A) as Release 10 (see Non-Patent Documents 5 and 6). TheLTE-A is based on the LTE radio communication system and is configuredby adding several new techniques to the system.

Carrier aggregation (CA) is studied for the LTE-A system, in which twoor more component carriers (CCs) are aggregated to support widertransmission bandwidths up to 100 MHz.

In the case where CA is configured, a UE has a single RRC connectionwith a network (NW). In RRC connection, one serving cell provides NASmobility information and security input. This cell is referred to as aprimary cell (PCell). In downlink, a carrier corresponding to PCell is adownlink primary component carrier (DL PCC). In uplink, a carriercorresponding to PCell is an uplink primary component carrier (UL PCC).

A secondary cell (SCell) is configured to form and a serving cell groupwith a PCell, in accordance with the UE capability. In downlink, acarrier corresponding to SCell is a downlink secondary component carrier(DL SCC). In uplink, a carrier corresponding to SCell is an uplinksecondary component carrier (UL SCC).

A serving cell group of one PCell and one or more SCells is configuredfor one UE.

The new techniques in the LTE-A include the technique of supportingwider bands (wider bandwidth extension) and the coordinated multiplepoint transmission and reception (CoMP) technique. The CoMP studied forLTE-A in 3GPP is described in Non-Patent Document 7.

The traffic flow of a mobile network is on the rise, and thecommunication rate is also increasing. It is expected that thecommunication rate will be further increased when the operations of theLTE and the LTE-A are fully initiated, leading to an increase in trafficflow.

Widespread use of smartphones and tablet terminals explosively increasestraffic in cellular radio communications, causing a fear of insufficientradio resources all over the world.

To deal with the problem of increased traffic, 3GPP is developingspecifications of Release 12. In the specifications of Release 12, theuse of small eNBs is studied to satisfy a tremendous volume of trafficin the future. In an example technique under study, a large number ofsmall eNBs are installed to configure a large number of small cells,thus increasing spectral efficiency for increased communicationcapacity.

In Release 12, dual connectivity is discussed as the technique ofconnecting a user equipment to both of a macro cell and a small cellwhen the macro cell and the small cell overlap each other (seeNon-Patent Document 11).

PRIOR ART DOCUMENTS Non-Patent Documents

Non-Patent Document 1: 3GPP TS 36.300 V11.7.0

Non-Patent Document 2: 3GPP TS 36.304 V11.2.0

Non-Patent Document 3: 3GPP S1-083461

Non-Patent Document 4: 3GPP R2-082899

Non-Patent Document 5: 3GPP TR 36.814 V9.0.0

Non-Patent Document 6: 3GPP TR 36.912 V10.0.0

Non-Patent Document 7: 3GPP TR 36.819 V11.1.0

Non-Patent Document 8: 3GPP TS 36.141 V11.1.0

Non-Patent Document 9: 3GPP R1-134496

Non-Patent Document 10: 3GPP R1-132236

Non-Patent Document 11: 3GPP TR 36.842 V0.2.0

SUMMARY OF INVENTION Problem to be Solved by the Invention

As described above, Non-Patent Document 11 discloses dual connectivityas the technique of connecting a user equipment to both of a macro celland a small cell when the macro cell and the small cell overlap eachother.

Non-Patent Document 11, however, discloses nothing about an approach inwhich a user equipment during dual connectivity performs handover. Theconventional handover method, in which a user equipment is connected toa single cell, does not reflect the connections of the UE to a macrocell and a small cell in dual connectivity.

Thus, the conventional handover approach cannot be applied to a userequipment during dual connectivity without any contrivance.

The present invention has an object to provide a communication system inwhich a user equipment device being connected with both of a macro celland a small cell successfully performs handover between macro cells.

Means to Solve the Problem

A communication system of the present invention is a communicationsystem including a user equipment device and a base station deviceconfiguring cells capable of radio communication with the user equipmentdevice. The cells include a plurality of macro cells and a small cellthat have a coverage in which the plurality of macro cells and the smallcell are communicable with the user equipment device. The coverage ofeach of the plurality of macro cells is relatively large, and thecoverage of the small cell is relatively small. When being connected toone of the plurality of macro cells and to the small cell, the userequipment device performs a pre-handover process of disconnectingconnection with the small cell before a handover process of switching amacro cell connected with the user equipment device from a macro cellbeing a moving source to a macro cell being a moving destination alongwith moving of the user equipment device, and a post-handover process ofreestablishing the connection with the small cell after the handoverprocess.

A communication system according to the present invention is acommunication system including a user equipment device and a basestation device configuring cells capable of radio communication with theuser equipment device. The cells include a plurality of macro cells anda small cell that have a coverage in which the plurality of macro cellsand the small cell are communicable with the user equipment device. Thecoverage of each of the plurality of macro cells is relatively large,and the coverage of the small cell is relatively small. When the userequipment device is connected to one of the plurality of macro cells andto the small cell, upon activation of a handover process of switching amacro cell connected with the user equipment device from a macro cellbeing a moving source to a macro cell being a moving destination alongwith moving of the user equipment device, the small cell is notifiedthat the macro cell that controls the small cell is to be changed.

Effects of the Invention

The communication system of the present invention allows the userequipment device being connected with both of the macro cell and thesmall cell to perform handover between the macro cells.

These and other objects, features, aspects and advantages of the presentinvention will become more apparent from the following detaileddescription of the present invention when taken in conjunction with theaccompanying drawings.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a diagram illustrating the configuration of a radio frame foruse in an LTE communication system.

FIG. 2 is a block diagram showing the overall configuration of an LTEcommunication system 700 under discussion of 3GPP.

FIG. 3 is a block diagram showing the configuration of a user equipment71 shown in FIG. 2, which is a user equipment according to the presentinvention.

FIG. 4 is a block diagram showing the configuration of a base station 72shown in FIG. 2, which is a base station according to the presentinvention.

FIG. 5 is a block diagram showing the configuration of an MME accordingto the present invention.

FIG. 6 is a flowchart showing an outline from a cell search to an idlestate operation performed by a user equipment (UE) in the LTEcommunication system.

FIG. 7 shows the concept of a cell configuration when macro eNBs andsmall eNBs coexist.

FIG. 8 shows example coverages of eNBs in a communication system of afirst embodiment of the present invention.

FIG. 9 shows another example coverages of eNBs in the communicationsystem of the first embodiment of the present invention.

FIG. 10 shows an example sequence of a handover-related process in thecommunication system of the first embodiment of the present invention.

FIG. 11 shows an example sequence of a pre-HO process in step ST908 ofFIG. 10.

FIG. 12 shows an example sequence of a HO process in step ST928 of FIG.10.

FIG. 13 shows an example sequence of a post-HO process in step ST949 ofFIG. 10.

FIG. 14 shows an example sequence of a handover-related process in acommunication system of a first modification of the first embodiment ofthe present invention.

FIG. 15 shows an example sequence of a pre-HO process in step ST1009 ofFIG. 14.

FIG. 16 shows an example sequence of a post-HO process in step ST1010 ofFIG. 14.

FIG. 17 shows an example sequence of a handover-related process in acommunication system of a second embodiment of the present invention.

FIG. 18 shows the example sequence of the handover-related process inthe communication system of the second embodiment of the presentinvention.

FIG. 19 shows the example sequence of the handover-related process inthe communication system of the second embodiment of the presentinvention.

FIG. 20 shows an example sequence of a handover-related process in acommunication system of a third embodiment of the present invention.

FIG. 21 shows the example sequence of the handover-related process inthe communication system of the third embodiment of the presentinvention.

FIG. 22 shows the example sequence of the handover-related process inthe communication system of the third embodiment of the presentinvention.

FIG. 23 shows an example sequence of a handover-related process in acommunication system of a fourth embodiment of the present invention.

FIG. 24 shows the example sequence of the handover-related process inthe communication system of the fourth embodiment of the presentinvention.

FIG. 25 shows the example sequence of the handover-related process inthe communication system of the fourth embodiment of the presentinvention.

FIG. 26 shows an example sequence of a handover-related process in acommunication system of a fifth embodiment of the present invention.

FIG. 27 shows the example sequence of the handover-related process inthe communication system of the fifth embodiment of the presentinvention.

FIG. 28 shows the example sequence of the handover-related process inthe communication system of the fifth embodiment of the presentinvention.

FIG. 29 shows an example sequence of a MeNB HO process for EPS bearer #1in step ST2025 of FIG. 28.

FIG. 30 shows the example sequence of the MeNB HO process for EPS bearer#1 in step ST2025 of FIG. 28.

FIG. 31 shows an example status of data transmission and reception toand from a UE.

FIG. 32 shows an example sequence of a MeNB HO process for EPS bearer #1in a communication system of a sixth embodiment of the presentinvention.

FIG. 33 shows the example sequence of the MeNB HO process for EPS bearer#1 in the communication system of the sixth embodiment of the presentinvention.

FIG. 34 shows an example sequence of a MeNB HO process for EPS bearer #1in a communication system of a seventh embodiment of the presentinvention.

FIG. 35 shows the example sequence of the MeNB HO process for EPS bearer#1 in the communication system of the seventh embodiment of the presentinvention.

DESCRIPTION OF EMBODIMENTS First Embodiment

FIG. 2 is a block diagram showing an overall configuration of an LTEcommunication system 700, which is under discussion of 3GPP. FIG. 2 willbe described. A radio access network is referred to as an evolveduniversal terrestrial radio access network (E-UTRAN) 70. A userequipment device (hereinafter, referred to as a “user equipment (UE)”)71 that is a communication terminal device is capable of radiocommunication with a base station device (hereinafter, referred to as a“base station (E-UTRAN Node B: eNB)”) 72 and transmits and receivessignals through radio communication.

The E-UTRAN is composed of one or a plurality of base stations 72,provided that a control protocol for a user equipment 71 such as a radioresource control (RRC), and user planes such as a packet dataconvergence protocol (PDCP), radio link control (RLC), medium accesscontrol (MAC), or physical layer (PHY) are terminated in the basestation 72.

The control protocol radio resource control (RRC) between the userequipment 71 and the base station 72 performs broadcast, paging, RRCconnection management, and the like. The states of the base station 72and the user equipment 71 in RRC are classified into RRC_IDLE andRRC_CONNECTED.

In RRC_IDLE, public land mobile network (PLMN) selection, systeminformation (SI) broadcast, paging, cell re-selection, mobility, and thelike are performed. In RRC_CONNECTED, the user equipment has RRCconnection and is capable of transmitting and receiving data to and froma network. In RRC_CONNECTED, for example, handover (HO) and measurementof a neighbor cell are performed.

The base stations 72 are classified into eNBs 76 and Home-eNBs 75. Thecommunication system 700 includes an eNB group 72-1 including aplurality of eNBs 76 and a Home-eNB group 72-2 including a plurality ofHome-eNBs 75. A system, composed of an evolved packet core (EPC) being acore network and an E-UTRAN 70 being a radio access network, is referredto as an evolved packet system (EPS). The EPC being a core network andthe E-UTRAN 70 being a radio access network may be collectively referredto as a “network”.

The eNB 76 is connected to an MME/S-GW unit (hereinafter, also referredto as an “MME unit”) 73 including a mobility management entity (MME), aserving gateway (S-GW), or an MME and an S-GW by means of an S1interface, and control information is communicated between the eNB 76and the MME unit 73. A plurality of MME units 73 may be connected to oneeNB 76. The eNBs 76 are connected to each other by means of an X2interface, and control information is communicated between the eNBs 76.

The Home-eNB 75 is connected to the MME unit 73 by means of an S1interface, and control information is communicated between the Home-eNB75 and the MME unit 73. A plurality of Home-eNBs 75 are connected to oneMME unit 73. Or, the Home-eNBs 75 are connected to the MME units 73through a Home-eNB gateway (HeNBGW) 74. The Home-eNB 75 is connected tothe HeNBGW 74 by means of an S1 interface, and the HeNBGW 74 isconnected to the MME unit 73 by means of an S1 interface.

One or a plurality of Home-eNBs 75 are connected to one HeNBGW 74, andinformation is communicated therebetween through an S1 interface. TheHeNBGW 74 is connected to one or a plurality of MME units 73, andinformation is communicated therebetween through an S1 interface.

The MME units 73 and HeNBGW 74 are entities of higher layer,specifically, higher nodes, and control the connections between the userequipment (UE) 71 and the eNB 76 and the Home-eNB 75 being basestations. The MME units 73 configure an EPC being a core network. Thebase station 72 and the HeNBGW 74 configure an E-UTRAN 70.

Further, 3GPP is studying the configuration below. The X2 interfacebetween the Home-eNBs 75 is supported. In other words, the Home-eNBs 75are connected to each other by means of an X2 interface, and controlinformation is communicated between the Home-eNBs 75. The HeNBGW 74appears to the MME unit 73 as the Home-eNB 75. The HeNBGW 74 appears tothe Horne-eNB 75 as the MME unit 73.

The interfaces between the Home-eNBs 75 and the MME units 73 are thesame, which are the S1 interfaces, in both cases where the Home-eNB 75is connected to the MME unit 73 through the HeNBGW 74 and it is directlyconnected to the MME unit 73.

The base station device 72 may configure a single cell or a plurality ofcells. Each cell has a range predetermined as a coverage in which thecell can communicate with a communication terminal device 71 andperforms radio communication with the communication terminal device 71within the coverage. In the case where one base station device 72configures a plurality of cells, every cell is configured so as tocommunicate with the user equipment 71.

FIG. 3 is a block diagram showing the configuration of the userequipment 71 of FIG. 2 that is a user equipment according to the presentinvention. The transmission process of the user equipment 71 shown inFIG. 3 will be described. First, a transmission data buffer unit 803stores the control data from a protocol processing unit 801 and the userdata from an application unit 802. The data stored in the transmissiondata buffer unit 803 is passed to an encoding unit 804 and is subjectedto an encoding process such as error correction. There may exist thedata output from the transmission data buffer unit 803 directly to amodulating unit 805 without the encoding process. The data encoded bythe encoding unit 804 is modulated by the modulating unit 805. Themodulated data is converted into a baseband signal, and the basebandsignal is output to a frequency converting unit 806 and is thenconverted into a radio transmission frequency. After that, atransmission signal is transmitted from an antenna 807 to the basestation 72.

The user equipment 71 executes the reception process as follows. Theradio signal from the base station 72 is received through the antenna807. The received signal is converted from a radio reception frequencyinto a baseband signal by the frequency converting unit 806 and is thendemodulated by a demodulating unit 808. The demodulated data is passedto a decoding unit 809 and is subjected to a decoding process such aserror correction. Among the pieces of decoded data, the control data ispassed to the protocol processing unit 801, and the user data is passedto the application unit 802. A series of processes by the user equipment71 is controlled by a control unit 810. This means that, though notshown in FIG. 3, the control unit 810 is connected to the individualunits 801 to 809.

FIG. 4 is a block diagram showing the configuration of the base station72 of FIG. 2 that is a base station according to the present invention.The transmission process of the base station 72 shown in FIG. 4 will bedescribed. An EPC communication unit 901 performs data transmission andreception between the base station 72 and the EPC (such as the MIME unit73), HeNBGW 74, and the like. A communication with another base stationunit 902 performs data transmission and reception to and from anotherbase station. The EPC communication unit 901 and the communication withanother base station unit 902 each transmit and receive information toand from a protocol processing unit 903. The control data from theprotocol processing unit 903, and the user data and the control datafrom the EPC communication unit 901 and the communication with anotherbase station unit 902 are stored in a transmission data buffer unit 904.

The data stored in the transmission data buffer unit 904 is passed to anencoding unit 905 and is then subjected to an encoding process such aserror correction. There may exist the data output from the transmissiondata buffer unit 904 directly to a modulating unit 906 without theencoding process. The encoded data is modulated by the modulating unit906. The modulated data is converted into a baseband signal, and thebaseband signal is output to a frequency converting unit 907 and is thenconverted into a radio transmission frequency. After that, atransmission signal is transmitted from an antenna 908 to one or aplurality of user equipments 71.

The reception process of the base station 72 is executed as follows. Aradio signal from one or a plurality of user equipments 71 is receivedthrough the antenna 908. The received signal is converted from a radioreception frequency into a baseband signal by the frequency convertingunit 907, and is then demodulated by a demodulating unit 909. Thedemodulated data is passed to a decoding unit 910 and is then subjectedto a decoding process such as error correction. Among the pieces ofdecoded data, the control data is passed to the protocol processing unit903, the EPC communication unit 901, or the communication with anotherbase station unit 902, and the user data is passed to the EPCcommunication unit 901 and the communication with another base stationunit 902. A series of processes by the base station 72 is controlled bya control unit 911. This means that, though not shown in FIG. 4, thecontrol unit 911 is connected to the individual units 901 to 910.

FIG. 5 is a block diagram showing the configuration of the MME accordingto the present invention. FIG. 5 shows the configuration of an MME 73 aincluded in the MME unit 73 shown in FIG. 2 described above. A PDN GWcommunication unit 1001 performs data transmission and reception betweenthe MME 73 a and the PDN GW. A base station communication unit 1002performs data transmission and reception between the MME 73 a and thebase station 72 by means of the S1 interface. In the case where the datareceived from the PDN GW is user data, the user data is passed from thePDN GW communication unit 1001 to the base station communication unit1002 via a user plane communication unit 1003 and is then transmitted toone or a plurality of base stations 72. In the case where the datareceived from the base station 72 is user data, the user data is passedfrom the base station communication unit 1002 to the PDN GWcommunication unit 1001 via the user plane communication unit 1003 andis then transmitted to the PDN GW.

In the case where the data received from the PDN GW is control data, thecontrol data is passed from the PDN GW communication unit 1001 to acontrol plane control unit 1005. In the case where the data receivedfrom the base station 72 is control data, the control data is passedfrom the base station communication unit 1002 to the control planecontrol unit 1005.

A HeNBGW communication unit 1004 is provided in the case where theHeNBGW 74 is provided, which performs data transmission and receptionbetween the MME 73 a and the HeNBGW 74 by means of the interface (IF)according to an information type. The control data received from theHeNBGW communication unit 1004 is passed from the HeNBGW communicationunit 1004 to the control plane control unit 1005. The processing resultsof the control plane control unit 1005 are transmitted to the PDN GW viathe PDN GW communication unit 1001. The processing results of thecontrol plane control unit 1005 are transmitted to one or a plurality ofbase stations 72 by means of the S1 interface via the base stationcommunication unit 1002, and are transmitted to one or a plurality ofHeNBGWs 74 via the HeNBGW communication unit 1004.

The control plane control unit 1005 includes a NAS security unit 1005-1,an SAE bearer control unit 1005-2, and an idle state mobility managingunit 1005-3, and performs an overall process for the control plane. TheNAS security unit 1005-1 provides, for example, security of a non-accessstratum (NAS) message. The SAE bearer control unit 1005-2 manages, forexample, a system architecture evolution (SAE) bearer. The idle statemobility managing unit 1005-3 performs, for example, mobility managementof an idle state (LTE-IDLE state, which is merely referred to as idle aswell), generation and control of a paging signal in the idle state,addition, deletion, update, and search of a tracking area of one or aplurality of user equipments 71 being served thereby, and tracking arealist management.

The MME 73 a distributes a paging signal to one or a plurality of basestations 72. In addition, the MME 73 a performs mobility control of anidle state. When the user equipment is in the idle state and an activestate, the MME 73 a manages a list of tracking areas. The MME 73 abegins a paging protocol by transmitting a paging message to the cellbelonging to a tracking area in which the UE is registered. The idlestate mobility managing unit 1005-3 may manage the CSG of the Home-eNBs75 to be connected to the MME 73 a, CSG-IDs, and a whitelist.

An example of a cell search method in a mobile communication system willbe described next. FIG. 6 is a flowchart showing an outline from a cellsearch to an idle state operation performed by a user equipment (UE) inthe LTE communication system. When starting a cell search, in Step ST1,the user equipment synchronizes slot timing and frame timing by aprimary synchronization signal (P-SS) and a secondary synchronizationsignal (S-SS) transmitted from a neighbor base station.

The P-SS and S-SS are collectively referred to as a synchronizationsignals (SSs). Synchronization codes, which correspond one-to-one toPCIs assigned per cell, are assigned to the synchronization signals(SSs). The number of PCIs is currently studied in 504 ways. The 504 waysof PCIs are used for synchronization, and the PCIs of the synchronizedcells are detected (specified).

In Step ST2, next, the user equipment detects a cell-specific referencesignal (CRS) being a reference signal (RS) transmitted from the basestation per cell and measures the reference signal received power(RSRP). The codes corresponding one-to-one to the PCTs are used for thereference signal RS. Separation from another cell is enabled bycorrelation using the code. The code for RS of the cell is derived fromthe PCI specified in Step ST1, so that the RS can be detected and the RSreceived power can be measured.

In Step ST3, next, the user equipment selects the cell having the bestRS received quality, for example, the cell having the highest RSreceived power, that is, the best cell, from one or more cells that havebeen detected up to Step ST2.

In Step ST4, next, the user equipment receives the PBCH of the best celland obtains the BCCH that is the broadcast information. A masterinformation block (MIB) containing the cell configuration information ismapped to the BCCH over the PBCH. Accordingly, the MIB is obtained byobtaining the BCCH through reception of the PBCH. Examples of the MIBinformation include the downlink (DL) system bandwidth (also referred toas a transmission bandwidth configuration (dl-bandwidth)), the number oftransmission antennas, and a system frame number (SFN).

In Step ST5, next, the user equipment receives the DL-SCH of the cellbased on the cell configuration information of the MIB, to therebyobtain a system information block (SIB) 1 of the broadcast informationBCCH. The SIB1 contains the information about the access to the cell,information about cell selection, and scheduling information on anotherSIB (SIBk; k is an integer equal to or greater than two). In addition,the SIB1 contains a tracking area code (TAC).

In Step ST6, next, the user equipment compares the TAC of the SIB1received in Step ST5 with the TAC portion of a tracking area identity(TAI) in the tracking area list that has already been possessed by theuser equipment. The tracking area list is also referred to as a TAIlist. TAI is the identification information for identifying trackingareas and is composed of a mobile country code (MCC), a mobile networkcode (MNC), and a tracking area code (TAC). MCC is a country code. MNCis a network code. TAC is the code number of a tracking area.

If the result of the comparison of Step ST6 shows that the TAC receivedin Step ST5 is identical to the TAC included in the tracking area list,the user equipment enters an idle state operation in the cell. If thecomparison shows that the TAC received in Step ST5 is not included inthe tracking area list, the user equipment requires a core network (EPC)including MIME and the like to change a tracking area through the cellfor performing tracking area update (TAU).

The device configuring a core network (hereinafter, also referred to asa “core-network-side device”) updates the tracking area list based on anidentification number (such as UE-ID) of a user equipment transmittedfrom the user equipment together with a TAU request signal. Thecore-network-side device transmits the updated tracking area list to theuser equipment. The user equipment rewrites (updates) the TAC list ofthe user equipment based on the received tracking area list. After that,the user equipment enters the idle state operation in the cell.

Widespread use of smartphones and tablet terminals explosively increasestraffic in cellular radio communications, causing a fear of insufficientradio resources all over the world. To increase spectral efficiency,thus, it is studied to downsize cells for further spatial separation.

In the conventional configuration of cells, the cell configured by aneNB has a relatively-wide-range coverage. Conventionally, cells areconfigured such that relatively-wide-range coverages of a plurality ofcells configured by a plurality of macro eNBs cover a certain area.

When cells are downsized, the cell configured by an eNB has anarrow-range coverage compared with the coverage of a cell configured bya conventional eNB. Thus, in order to cover a certain area as in theconventional case, a larger number of downsized eNBs than theconventional eNBs are required.

In the description below, a “macro cell” refers to a cell whose coverageis relatively large, such as a cell configured by a conventional eNB,and a “macro eNB” refers to an eNB configuring a macro cell. A “smallcell” refers to a cell whose coverage is relatively small, such as adownsized cell, and a “small eNB” refers to an eNB configuring a smallcell.

The macro eNB may be, for example, a “wide area base station” describedin Non-Patent Document 8.

The small eNB may be, for example, a low power node, local area node, orhotspot. Alternatively, the small eNB may be a pico eNB configuring apico cell, a femto eNB configuring a femto cell, HeNB, remote radio head(RRH), remote radio unit (RRU), remote radio equipment (RRE), or relaynode (RN). Still alternatively, the small eNB may be a “local area basestation” or “home base station” described in Non-Patent Document 8.

FIG. 7 shows the concept of the cell configuration in which macro eNBsand small eNBs coexist. The macro cell configured by a macro eNB has arelatively-wide-range coverage 1301. A small cell configured by a smalleNB has a coverage 1302 whose range is smaller than that of the coverage1301 of the macro cell configured by a macro eNB.

When a plurality of eNBs coexist, the coverage of the cell configured byan eNB may be included in the coverage of the cell configured by anothereNB. In the cell configuration shown in FIG. 7, as indicated by areference “1304” or “1305”, the coverage 1302 of the small cellconfigured by a small eNB may be included in the coverage 1301 of themacro cell configured by a macro eNB.

As indicated by a reference “1305”, the coverages 1302 of a pluralityof, for example, two small cells may be included in the coverage 1301 ofone macro cell. A user equipment (UE) 1303 is included in, for example,the coverage 1302 of the small cell and performs communication via thesmall cell.

In the cell configuration shown in FIG. 7, as indicated by a reference“1306”, the coverage 1301 of the macro cell configured by a macro eNBmay overlap the coverages 1302 of the small cells configured by smalleNBs in a complicated manner.

As indicated by a reference “1307”, the coverage 1301 of the macro cellconfigured by a macro eNB may not overlap the coverages 1302 of thesmall cells configured by small eNBs.

Further, as indicated by a reference “1308”, the coverages 1302 of alarge number of small cells configured by a large number of small eNBsmay be configured in the coverage 1301 of one macro cell configured byone macro eNB.

FIGS. 8 and 9 each show example coverages of eNBs in a communicationsystem of a first embodiment of the present invention. FIGS. 8 and 9each show the case where a UE 57 during dual connectivity performs HObetween macro cells 51 and 53.

In the following description, a macro cell that performs dualconnectivity may be referred to as a “master cell”, and an eNB thatconfigures the master cell may be referred to as a “master eNB(abbreviated as MeNB)”. A MeNB that is a HO source may be referred to asa “source MeNB (abbreviated as S-MeNB)”, and a MeNB that is a HOdestination may be referred to as a “target MeNB (abbreviated asT-MeNB)”.

A small cell that performs dual connectivity may be referred to as a“secondary cell”, and an eNB that configures the secondary cell may bereferred to as a “secondary eNB (abbreviated as SeNB)”.

In FIGS. 8 and 9, the S-MeNB is denoted by a reference “51”, and thecoverage of the S-MeNB 51 is denoted by a reference “52”. The T-MeNB isdenoted by a reference “53”, and the coverage of the T-MeNB 53 isdenoted by a reference “54”. The SeNB is denoted by a reference “55”,and the coverage of the SeNB 55 is denoted by a reference “56”.

FIG. 9 shows a case where another SeNB 58 is present in addition to theSeNB 55 shown in FIG. 8. In FIG. 9, the SeNB 55 shown in FIG. 8 is aSeNB that is a moving source (hereinafter, also referred to as a“moving-source SeNB”) 55, and the other SeNB is a SeNB that is a movingdestination (hereinafter, also referred to as a “moving-destinationSeNB”) 58. In FIG. 9, a reference “56” denotes the coverage of themoving-source SeNB 55, and a reference “59” denotes the coverage of themoving-destination SeNB 58.

This embodiment will describe a case where a measurement report isperformed, which indicates that moving of the UE 57 as shown in FIGS. 8and 9 has degraded the reception quality of the S-MeNB 51 and hasimproved the reception quality of the T-MeNB 53 in the measurement ofneighbor cells by the UE 57.

FIG. 10 shows an example sequence of a handover-related process in thecommunication system of the first embodiment of the present invention.FIG. 11 shows an example sequence of a pre-HO process in step ST908 ofFIG. 10. FIG. 12 shows an example sequence of a HO process in step ST928of FIG. 10. FIG. 13 shows an example sequence of a post-HO process instep ST949 of FIG. 10. Here, the handover-related process refers to theprocess related to handover (HO) and includes the pre-HO process, the HOprocess, and the post-HO process.

This embodiment will disclose a method in which a UE during dualconnectivity performs HO between macro cells in the case of Alternative1A of a user plane architecture of dual connectivity described inNon-Patent Document 11 (see 8.1.1.1 of Non-Patent Document 11).

In Alternative 1A of the user plane architecture, communications areperformed through a path through which communication is performed froman S-GW via a MeNB (hereinafter, also referred to as a “via-MeNB path”)and through a path through which communication is performed from an S-GWvia a SeNB that is a small cell (hereinafter, also referred to as a“via-SeNB path”). The via-MeNB path is a path using the bearer 1 and isused in communications of packet data in, for example, steps ST902 andST903. The via-SeNB path is a path using the bearer 2 and is used incommunications of packet data in, for example, steps ST904 and ST905.

Disclosed below is a method in which a MeNB notifies a UE during dualconnectivity of a measurement control message.

In the example shown in FIG. 10, in step ST901, the S-MeNB notifies theUE of a measurement control message. Measurement of a neighbor SeNB maybe configured in the measurement control message. Measurement of afrequency for SeNB may be configured. Or, as the configuration of ameasurement, an event for SeNB or an event at a frequency for SeNB, orevent criteria may be configured separately from that for MeNB.

Examples of configuration parameters include a SeNB identifier, afrequency, an event number for report, a threshold of reception quality,and a measurement period. Examples of the reception quality includereference signal received power (RSRP) and reference signal receivedquality (RSRQ).

The UE that has received the measurement control message in step ST901measures neighbor cells (MeNB and SeNB).

In step ST906, the UE notifies the S-MeNB of a measurement reportmessage. In step ST907, the S-MeNB that has received the measurementreport message in step ST906 uses the result of the measurement reportto decide whether to cause the UE to perform handover (HO) to theT-MeNB. In the example shown in FIG. 10, in step ST907, the S-MeNBdecides to cause the UE to perform HO to the T-MeNB.

When the S-MeNB decides to cause the UE to perform HO to the T-MeNB instep ST907, the process moves to step ST908. In step ST908, a SeNBrelease process that is the pre-HO process shown in FIG. 11 is executed.

Specifically, when deciding to cause the UE to perform HO to the T-MeNBin step ST907 of FIG. 10, the S-MeNB moves to step ST909 of FIG. 11.

In step ST909, the S-MeNB notifies the SeNB of a SeNB release requestmessage. In step ST910, the SeNB that has received the SeNB releaserequest message in step ST909 notifies the S-MeNB of a SeNB releaseresponse message.

In step ST911, the S-MeNB notifies the UE of an RRC connectionreconfiguration message as the RRC-related information.

In step ST912, the SeNB performs SN status transfer to transfer thestatus of the sequence number (abbreviated as SN) of the PDCP to theS-MeNB. Specifically, the SeNB notifies the S-MeNB of the PDCP SNinformation. In step ST913, the SeNB may perform data forwarding toforward the yet-to-be-transmitted data to the S-MeNB, therebyestablishing loss-free communication.

The SeNB may determine whether to perform forwarding or whether toperform reordering in accordance with a bearer type, quality of service(abbreviated as QoS), a delay time occurring in a backhaul of a cellbeing a connection destination, or the number of forwardings(reforwardings of forwarded data). For example, the SeNB does notperform forwarding or reordering for the services that cause no problemin the event of data loss, such as voice over internet protocol (VoIP).For example, the SeNB does not perform forwarding when forwarded data isto be reforwarded by the handover. Alternatively, for the services thatrequire real-time capability, the SeNB does not perform forwarding if adelay time occurring in a backhaul of a cell being a connectiondestination is large. As a result, resources can be releasedimmediately, thus providing a stable operation.

In step ST914, the S-MeNB stores the data forwarded from the SeNB in thebuffer.

In step ST915, the UE notifies the S-MeNB of an RRC connectionreconfiguration complete message. This completes RRC configuration andradio synchronization between the S-MeNB and the UE, so thatcommunication is started between the UE and the S-MeNB in step ST916,and also, communication is started between the UE and the S-GW in stepST917.

In step ST918, the S-MeNB notifies the MME of a path switch requestmessage requesting a path switch. In step ST919, the MME that has beennotified of the path switch request message notifies the S-GW of amodify bearer request message requesting a modification of a bearer.

In step ST918 or ST919, the information indicating the presence orabsence of a change of the SeNB may be provided. The informationindicating that the request is not a path switch request or a modifybearer request due to HO of the UE may be provided. Or, it may beindicated that, by providing no information for identifying a UE being aHO target, the relevant request is not a path switch request or a modifybearer request due to HO of the UE.

Consequently, differently from a normal HO process, the MME, the S-GW,or a location registration and management function unit separatelyprovided can omit a process of updating the location information of a UEand a process of managing radio resources associated with an update ofthe location information.

In step ST920, the S-GW that has been notified of the modify bearerrequest message changes the transmission destination of the bearer 2,which has been used in communication, from the SeNB to the S-MeNB. Also,in this case, the S-GW may modify the bearer for via-S-MeNB path inconsideration of the user accommodation status of the S-MeNB.

In step ST924, the S-GW notifies the M1VIE of a modify bearer responsemessage indicating that the S-GW has responded to the modify bearerrequest. In step ST925, the MME that has been notified of the modifybearer response message notifies the S-MeNB of a path switch request Ackmessage indicating that the MME has accepted the path switch request.

In this way, in steps ST902 and ST903 of FIG. 10 and steps ST916 andST922 of FIG. 11, the S-MeNB transmits and receives by two bearers,namely, bearer 1 and bearer 2, to and from the same UE.

In this case, in step ST921, the S-GW may provide an end marker to thePDCP to be transmitted to the SeNB, thus informing about the completionof the forwarding process. This allows the SeNB to recognize the end ofthe forwarded data, thus releasing a forwarding buffer at economicaltiming.

In step ST923, the SeNB may provide an end marker and forward it to theS-MeNB. This allows the S-MeNB to release a forwarding accept buffer ateconomical timing.

In step ST926, the S-MeNB notifies the SeNB of a UE context releasemessage instructing to release a UE context. When being notified of theend marker in step ST923, in step ST926, the S-MeNB notifies the SeNB ofthe UE context release message and also notifies the SeNB that theS-MeNB has received the end marker. By notifying the SeNB of the UEcontext release message or that the S-MeNB has received the end markerin this way, in step ST927, the SeNB can release the managementinformation of the UE.

As to the end marker, whether to provide an end marker may be determinedin accordance with a bearer type or QoS. As to the service that causesno problem in the event of data loss, such as voice data, for example,VoIP, resources can be managed immediately by releasing a resourcewithout providing an end marker to the data, leading to an effect that astable operation can be provided.

As shown in FIG. 8, when the UE 57 moves and approaches the boundary ofthe coverage 52 of the S-MeNB 51, in step ST907 of FIG. 10, the S-MeNB51 decides HO to the T-MeNB 53 based on the measurement report of the UE57. In this case, the SeNB 55 during communication has a goodmeasurement value.

When the SeNB 55 has a good measurement value, the S-MeNB 51 determinesthat the UE 57 is present within the coverage 56 of the SeNB 55 alsoafter HO. Then, in step ST909 of FIG. 11, the resource holdinginstruction information or the ID for holding a resource may be providedto the release request message to be notified to the SeNB 55 and thenmay be notified.

As a result, the SeNB 55 does not release the configuration orinformation related to the configured RRC and the configuration orinformation related to radio synchronization. After HO to the T-MeNB 53,accordingly, a resource reserving process and a resynchronizationprocess in reconfiguration of dual connectivity from the T-MeNB 53 tothe SeNB 55 can be eliminated.

After releasing the management information of the UE by the SeNB in stepST927, in step ST928 of FIG. 10, the process of handover from the S-MeNBto the T-MeNB is performed. This causes both of the two bearers to beswitched from the S-MeNB to the T-MeNB.

Specifically, the handover process of step ST928 is executed as shown inFIG. 12. In step ST929, the S-MeNB notifies the T-MeNB that is a HOdestination of a handover request message. The HO request messageincludes E-UTRAN radio access bearer (E-RAB) information for performingHO.

In step ST930, the T-MeNB performs admission control to confirmaccommodation capacity. When judging that it can accept HO based on theresult of the admission control, in step ST931, the T-MeNB notifies theS-MeNB of a handover request Ack message.

Upon receipt of the handover request Ack message of step ST931, in stepST932, the S-MeNB notifies the UE of an RRC connection reconfigurationmessage including mobility control information.

In step ST933, the S-MeNB performs SN status transfer to the T-MeNB.Specifically, the S-MeNB notifies the T-MeNB of the PDCP SN information.

In step ST934, the S-MeNB may perform data forwarding to forwardyet-to-be-transmitted data to the T-MeNB. In step ST935, the T-MeNBstores the data forwarded from the S-MeNB in the buffer.

In step ST936, the UE completes the radio configuration, and notifiesthe T-MeNB of an RRC connection reconfiguration complete message. Whenthe radio configuration completes in this way, communication is startedbetween the UE and the T-MeNB in step ST937, and also, communication isstarted between the UE and the S-GW in step ST938.

In step ST939, the T-MeNB notifies the MME of a path switch requestmessage. In step ST940, the MME that has been notified of the pathswitch request message notifies the S-GW of a modify bearer requestmessage.

The information indicating the presence or absence of a change of theSeNB may be provided in step ST939 or ST940. The information indicatingthat the relevant request is a path switch request or modify bearerrequest due to HO of the UE may be provided. The information foridentifying the UE that is a HO target may be provided to indicate thatthe relevant request is a path switch request or a modify bearer requestdue to HO of the UE. Or, the relevant request may be a conventional pathswitch request or a conventional modify bearer request to indicate theconventional HO of the UE.

This allows the MME, the S-GW, or the location registration andmanagement function unit separately provided to perform, as a normal HOprocess, the process of updating the location information of the UE andthe process of managing radio resources associated with the update ofthe location information.

In step ST941, the S-GW that has been notified of the modify bearerrequest message changes the transmission destination of the bearer 2used in communication to the T-MeNB. Also, in this case, the S-GW maymodify the bearer for via-T-MeNB path in consideration of the useraccommodation status of the T-MeNB.

The bearer 1 and the bearer 2 are configured for route via the T-MeNB,so that communication is performed between the UE and the T-MeNB in stepST937, and also, communication is performed between the S-GW and theT-MeNB in step ST943.

In step ST945, the S-GW notifies the MIME of a modify bearer responsemessage. In step ST946, the MME that has been notified of the modifybearer response message notifies the T-MeNB of a path switch request Ackmessage.

For loss-free transmission, the S-GW may provide an end marker to thefinal data to the S-MeNB in step ST942. This allows the S-MeNB torelease a forwarding buffer at economical timing.

In step ST944, the S-MeNB may provide an end marker to the finallyforwarded data. This allows the T-MeNB to release the forward acceptancebuffer at economical timing.

In step ST947, the T-MeNB notifies the S-MeNB of a UE context releasemessage. When being notified of the end marker in step ST944, in stepST947, the T-MeNB notifies the S-MeNB of a UE context release messageand also notifies the S-MeNB that the T-MeNB has received the endmarker. The S-MeNB is notified of the UE context release message ornotified that the T-MeNB has received the end marker in this way, thusallowing the S-MeNB to release the management information of the UE instep ST948.

After the S-MeNB releases the management information of the UE in stepST948, in step ST949 of FIG. 10, the SeNB addition process that is apost-HO process is performed. Specifically, the SeNB addition process ofstep ST949 is executed as shown in FIG. 13.

After the switch to the T-MeNB by the HO process in step ST928 of FIG.10, in step ST950 of FIG. 13, the T-MeNB notifies the SeNB of a SeNBaddition request message.

In step ST951, the SeNB notifies the T-MeNB of a SeNB addition responsemessage.

In step ST952, the T-MeNB notifies the UE of an RRC connectionreconfiguration message as the RRC-related information.

Upon the SeNB notifying of the SeNB addition response message indicatingthat the SeNB has received the SeNB addition request in response to theSeNB addition request message in step ST951, in step ST953, the T-MeNBnotifies the SeNB of the PDCP SN information as an SN status transfer.

In step ST954, the T-MeNB may perform data forwarding to forwardyet-to-be-transmitted data to the SeNB, thereby establishing loss-freecommunication.

The T-MeNB may determine whether to perform data forwarding or performSN reordering in accordance with a bearer type or QoS. For the bearerthat causes no problem in the event of data loss, such as voice data,for example, VoIP, eliminating data forwarding or SN reordering achievesthe effects that resources can be released immediately and a stableoperation can be provided.

In step ST955, the SeNB stores the data forwarded from the T-MeNB in thebuffer.

In step ST956, the UE completes radio synchronization and notifies theT-MeNB of an RRC connection reconfiguration complete message indicatingthat the reconfiguration of the RRC connection between the SeNB and theUE has completed. When the radio synchronization completes in this way,communication is started between the UE and the SeNB in step ST957, andalso, communication is started between the UE and the S-GW in stepST958.

Further, in step ST959, the SeNB notifies the T-MeNB of the completionof radio configuration, specifically, a SeNB addition complete messageindicating that the addition of the SeNB has completed. In step ST960,the T-MeNB that has been notified of the SeNB addition complete messagenotifies the MME of a path switch request message. In step ST961, theMIME that has been notified of the path switch request message notifiesthe S-GW of a modify bearer request message.

In step ST960 or ST961, the information indicating the presence orabsence of a change (addition) of the SeNB may be provided. Theinformation indicating that the request is not a path switch request ora modify bearer request due to HO of the UE may be provided. Or, it maybe indicated that, by providing no information for identifying a UEbeing a HO target, the relevant request is not a path switch request ora modify bearer request due to HO of the UE.

Consequently, differently from a normal HO process, the MME, the S-GW,or the location registration and management function unit separatelyprovided can omit a process of updating the location information of a UEand a process of managing radio resources associated with an update ofthe location information.

In step ST962, the S-GW that has been notified of the modify bearerrequest message changes the transmission destination of the bearer 2used in communication to the SeNB. Also, in this case, the S-GW maymodify a bearer for via-SeNB path in consideration of the useraccommodation status of the SeNB.

In step ST966, the S-GW notifies the MME of a modify bearer responsemessage. In step ST967, the MME that has been notified of the modifybearer response message notifies the T-MeNB of a path switch request Ackmessage indicating the completion of a path switch. As a result, thebearer 2 is configured for route via the SeNB, so that communication isperformed between the UE and the SeNB in step ST957, and also,communication is performed between the S-GW and the SeNB in step ST964.In step ST968, the T-MeNB may release the management information of theUE to the bearer 2.

In this case, in step ST963, the S-GW may provide an end marker to thePDCP to be transmitted to the T-MeNB, thus informing about thecompletion of the forwarding process. This allows the T-MeNB torecognize the end of the forwarded data, thus releasing a forwardingbuffer at economical timing.

In step ST965, the T-MeNB may provide an end marker to the data andforward the data to the SeNB. This allows the SeNB to release theforwarding buffer at economical timing.

As to the end marker, whether to provide an end marker may be determinedin accordance with a bearer type or QoS. As to the service that causesno problem in the event of data loss, such as voice data, for example,VoIP, resources can be managed immediately by releasing a resourcewithout providing an end marker to the data, leading to an effect that astable operation can be provided.

As shown in FIG. 9 described above, the SeNB 58 is added after the UE 57moves and approaches the boundary of the coverage 52 of the S-MeNB 51,the moving-source SeNB 55 is released, and HO from the S-MeNB 51 to theT-MeNB 53 is executed. At this time, the T-MeNB 53 may decide ato-be-added SeNB 58 in accordance with the moving direction informationof the UE 57, the moving speed information of the UE 57, the locationinformation of a neighbor SeNB (the GPS information of each SeNB or theinformation indicating to which MeNB each SeNB is closer), the cell sizeof a SeNB that is a moving destination (which may be a maximumtransmission power value of the SeNB corresponding to a cell size), theinformation indicating whether the UE is included in a closed subscribergroup (CSG), or a combination of these pieces of information, inaddition to the measurement report notified in step ST906 of FIG. 10.The UE described herein may be a mobile router that organizes aplurality of UEs. As the judgment result, the SeNB specificationinformation for specifying a SeNB to be added after HO may be added tothe HO request message notified in step ST929 of FIG. 12.

For example, in the case where a UE is on a train, even when themeasurement information of the UE shows that a measurement value on aSeNB during communication is better than that on a moving-destinationSeNB, the dual connectivity process can be alleviated by adding amoving-destination SeNB if the moving-destination SeNB has acommunicable quality and the location information shows that themoving-destination SeNB is within the coverage of the T-MeNB. In otherwords, an increase in process due to an immediate release of amoving-source SeNB and addition of a moving-desination SeNB by the UEmoving at high speed after the addition of the moving-soruce SeNB can beprevented.

For example, during a communication of a moving-source SeNB in a CSG,even when the moving-destination SeNB is temporarily inadvertently gooddue to shadowing by a human body, a moving-source SeNB is added becausethe communication is performed in the CSG or the moving speed is slow.This enables continuous, stable communication.

As described above, in this embodiment, when a UE is connected to onemacro cell and one small cell and performs dual connectivity, a pre-HOprocess is performed before a HO process of switching a macro cellconnected with the UE from the S-MeNB that is a macro cell being amoving source to the T-MeNB that is a macro cell being a movingdestination, and after the HO process, a post-HO process is performed.In the pre-HO process, the SeNB that is a small cell is released, thusdisconnecting the connection with the SeNB. In the post-HO process, aSeNB is added, thus reestablishing the connection with the SeNB.

This enables the UE during dual connectivity to execute HO between macrocells in the case of Alternative 1A of the user plane architecture ofdual connectivity described in Non-Patent Document 11.

First Modification of First Embodiment

FIG. 14 shows an example sequence of a handover-related process in acommunication system of a first modification of the first embodiment ofthe present invention. FIG. 15 shows an example sequence of a pre-HOprocess in step ST1009 of FIG. 14. FIG. 16 shows an example sequence ofa post-HO process in step ST1010 of FIG. 14. The handover-relatedprocess of this modification is similar to the handover-related processof the first embodiment shown in FIGS. 10 to 13 described above, andthus, the same steps will be denoted by the same step numbers, anddescription thereof will be omitted.

This embodiment will disclose a method in which a UE during dualconnectivity performs HO between macro cells in the case of Alternative3C of the user plane architecture of dual connectivity described inNon-Patent Document 11 (see 8.1.1.8 of Non-Patent Document 11).

In Alternative 3C of the user plane architecture, by the MeNB, betweenthe MeNB and the UE, bearer split where bearer (bearer 2) is split witha path for performing communciation via a small cell is performed, witha path (bearer 1) for performing direct communication.

In bearer split, an E-RAB corresponding to one EPS bearer is separatedinto two paths by an S-MeNB, so that data is communicated between theS-GW and the UE. Through one path, in step ST1001, data is directlycommunicated between the S-MeNB and the UE. Through the other path, insteps ST1003 and ST1004, data is communicated between the S-MeNB and theUE via the SeNB.

Between the S-MeNB and the S-GW, in step ST1002, data is communicatedthrough one path. The communication between the SeNB and the MeNB isperformed between different eNBs. If the communication quality of achannel between the different eNBs is poor, for example, dropped datamay occur. In order to solve such a problem, a deliver confirmationprocess may be introduced in the communication between the SeNB and theMeNB. For example, a retransmission process may be introduced. Thisreduces dropped data in the communication between the SeNB and the MeNB.

The case where a UE during dual connectivity performs HO between MeNBsby bearer split will be described.

Described below is a case where moving of a UE degrades the receptionquality of an S-MeNB and improves the reception quality of a T-MeNB inthe measurement, and a measurement report is performed in accordancewith event criteria.

In this embodiment, a SeNB release process is performed as describedbelow. In step ST907 of FIG. 15, the S-MeNB that has received themeasurement report from the UE in step ST906 of FIG. 14 uses the resultof the measurement report to decide to cause the UE to perform HO to theT-MeNB.

When deciding to cause the UE to perform HO to the T-MeNB, in stepST909, the S-MeNB transmits a SeNB release request message requestingrelease of the SeNB to the SeNB. In step ST911, the S-MeNB transmits, asthe RRC-related information, an RRC connection reestablishment requestmessage to the UE. This causes the S-MeNB to stop bearer split of thebearer 2 and transmit and receive the bearer 2 directly to and from theUE in steps ST916 and ST917.

After the release of the SeNB, in step ST928 of FIG. 14, the handover(HO) process from the S-MeNB to the T-MeNB is performed. The HO processof step ST928 is executed similarly to the HO process shown in FIG. 12described above. This HO process switches both of the two bearers fromthe S-MeNB to the T-MeNB.

After the switch to the T-MeNB, the post-HO process in step ST1010 ofFIG. 14 is executed. Specifically, in step ST950 of FIG. 16, the T-MeNBnotifies the SeNB of an addition request message, and in step ST952,notifies the UE of the RRC-related information for communication.Consequently, the bearer 2 is subjected to bearer split.

One bearer of the bearer 2 subjected to bearer split is used when datais directly communicated between the T-MeNB and the UE in step ST1005,and the other bearer is used when data is communicated between theT-MeNB and the UE via the SeNB in steps ST1007 and ST1008. Between theT-MeNB and the S-GW, in step ST1006, data is communicated through onepath. In this way, bearer split can be processed without sending a pathswitch request to the MME and the S-GW.

As described above, this modification enables the UE during dualconnectivity to perform HO between macro cells in the case ofAlternative 3C of the user plane architecture of dual connectivitydescribed in Non-Patent Document 11, that is, in the case wherecommunication is performed between the macro cell and the UE and betweenthe small cell and the UE using a bearer subjected to bearer split.

Second Embodiment

FIGS. 17 to 19 show an example sequence of a handover-related process ina communication system of a second embodiment of the present invention.FIG. 17 is continuous with FIG. 18 at a boundary BL1. FIG. 18 iscontinuous with FIG. 19 at a boundary BL2. The handover-related processof this embodiment is similar to the handover-related process of thefirst embodiment shown in FIGS. 10 to 13 described above, and thus, thesame steps will be denoted by the same step numbers and commondescription will be omitted.

This embodiment will disclose a method of performing HO withoutreleasing a SeNB in the case of Alternative 1A of the user planearchitecture of dual connectivity described in Non-Patent Document 11(see 8.1.1.1 of Non-Patent Document 11).

In Alternative 1A of the user plane architecture, in steps ST902 andST903, communication is performed through two paths: a path throughwhich communication is performed from the S-GW via the MeNB using thebearer 1 in steps ST902 and ST903, and a path through whichcommunication is performed from the S-GW via the small cell using thebearer 2 in steps ST904 and ST905.

In this embodiment, the connection of the UE during dual connectivitywith the SeNB is not released when the UE performs HO between MeNBs. Inother words, HO between MeNBs is performed while keeping dualconnectivity of the UE.

Disclosed below is a method in which a MeNB notifies a UE during dualconnectivity of a measurement control message.

In the example shown in FIGS. 17 to 19, in step ST901, the S-MeNBnotifies the UE of a measurement control message. Measurement ofneighbor SeNBs may be configured in the measurement control message.Measurement of a frequency for SeNB may be configured. Or, as themeasurement configuration, an event for SeNB or an event at a frequencyfor SeNB, or event criteria may be configured separately from that forMeNB.

Examples of the configuration parameter include a SeNB identifier, afrequency, an event number for report, a threshold of reception quality,and a measurement period. Examples of the reception quality include RSRPand RSRQ.

The UE that has received the measurement control message in step ST901measures the MeNB and the SeNB.

Described below is a case where moving of a UE degrades the receptionquality of an S-MeNB and improves the reception quality of a T-MeNB inthe measurement, and a measurement report is performed in accordancewith event criteria.

In step ST906, the UE performs a measurement report to the S-MeNB. Themeasurement report from the UE may include the identifier and thereception quality of the SeNB used for dual connectivity.

In step ST907, the S-MeNB that has received the measurement report fromthe UE in step ST906 uses the result of the measurement report to decideto cause the UE to perform HO to the T-MeNB.

In step ST1100, the S-MeNB notifies the T-MeNB of a HO request message.The information related to a SeNB (hereinafter also referred to as “SeNBinformation”) may be included in the HO request message to be notified.Examples of the SeNB information include a SeNB identifier and thereception quality of the SeNB in the UE.

A non-limiting example of the information related to the bearer of theSeNB (hereinafter also referred to as “bearer information”) is an E-RABidentifier. When a plurality of bearers are configured in a SeNB, theMeNB may provide an identifier to each bearer. The identifier of eachbearer and the information related to a bearer configuration may benotified in association with each other. The identifier of the SeNB andthe identifier of the bearer may be notified in association with eachother. Examples of the information related to the bearer configurationconfigured per bearer include QoS parameters, specifically, a quality ofservice (QoS) class identifier (abbreviated as QCI), allocation andretention priority (abbreviated as ARP), and guaranteed bit rate QoSinformation.

This enables a configuration per bearer, so that the S-MeNB can notifythe T-MeNB of a bearer configuration used by the SeNB. The T-MeNB canobtain a bearer configuration for each bearer of the SeNB, and thus candetermine whether the bearer configuration needs to be changed perbearer. The bearer configuration may be notified in association with thebearer information of a SeNB.

A non-limiting example of the configuration information related to RRCis an RRC context. Examples of the RRC context include an access stratum(AS) configuration indicating the configuration of a radio resource anda radio resource management (abbreviated as RRM) configurationindicating RRM information.

The RRC-related information includes the RRC-related information forMeNB and the RRC-related information for SeNB. Each piece of RRC-relatedinformation may be discernable. For example, an RRC context for MeNB andan RRC context for SeNB are created separately. Alternatively, theinformation for MeNB and the information for SeNB may be createdseparately in one RRC context.

Consequently, the RRC-related information for SeNB and the RRC-relatedinformation for MeNB are configurable dedicatedly, so that the S-MeNBcan notify the T-MeNB of the RRC-related information of each of the MeNBand the SeNB. The T-MeNB can obtain the RRC-related information of eachof the S-MeNB and the SeNB. The T-MeNB can thus determine whether theRRC-related configuration of the SeNB needs to be changed.

In step ST1101, the T-MeNB determines whether to change the SeNB usingthe information received in the HO request message from the S-MeNB. Thisprocess determines whether the bearer of the SeNB can be handed over tothe T-MeNB in the same state as it was configured in the S-MeNB. Forexample, in some cases, the number of S-MeNBs that can be accommodatedby the T-MeNB is limited, and a S-MeNB cannot be accepted due to such alimit. In the case of Alternative 3C of the user plane architecture indual connectivity, whether a bearer subjected to bearer split by theS-MeNB can be accommodated is also the information for judgment.

If judging to change the SeNB, the T-MeNB moves to step ST1102. Or, theT-MeNB may move to step ST1 102 when deciding to release the SeNB in theHO process, not limited to the case where the T-MeNB determines tochange the SeNB.

In step ST1102, the T-MeNB executes the SeNB release process of stepST908 shown in FIG. 10 to release the SeNB, and then, executes the HOprocess of step ST908. Specifically, the T-MeNB notifies the S-MeNB ofthe information indicating a request to execute the SeNB release process(hereinafter also referred to as “SeNB release process executing requestinformation”). The S-MeNB that has been notified of the SeNB releaseprocess execution request information from the T-MeNB may apply themethod disclosed in the first embodiment, description of which will beomitted here.

In this case, the HO request from the S-MeNB to the T-MeNB in stepST1100 may be omitted. After notifying the S-MeNB of the SeNB releaseprocess execution request information, the T-MeNB may perform admissioncontrol of step ST930.

The T-MeNB may use the handover request Ack message to notify the S-MeNBof the SeNB release process execution request information. For example,the T-MeNB may notify of the information using the handover request Ackmessage in step ST1104. The T-MeNB that has decided to release the SeNBin the HO process determines to perform HO only between MeNBs, performsadmission control based on the judgment, includes the SeNB releaseprocess execution request information in a handover request Ack message,and notifies the S-MeNB of the handover request Ack message. When thesystem is defined such that the SeNB is released in HO of the MeNB, thehandover request Ack message may indicate a request to execute a SeNBrelease process. The S-MeNB performing dual connectivity that hasreceived the handover request Ack message from the T-MeNB executes theSeNB release process.

The T-MeNB may notify the UE of SeNB release process execution requestinformation using the RRC connection reconfiguration message includingmobility control information (MCI). For example, an RRC connectionreconfiguration message including MCI of step ST1106 may be used. Whenthe system is defined such that the SeNB is released in HO of the MeNB,the RRC connection reconfiguration message including MCI may indicatethe request to execute a SeNB release process. The UE configuring theSeNB that has received the RRC connection reconfiguration messageincluding MCI from the S-MeNB executes the process of releasing theSeNB.

If judging not to change the SeNB, in step ST1103, the T-MeNB decides toperform the HO process while continuously using the SeNB. In otherwords, the T-MeNB only changes the MeNB. If judging not to change theSeNB, the T-MeNB does not perform the process of changing the bearer 2being used by the SeNB.

In step ST930, the T-MeNB performs admission control in response to a HOrequest. The admission control may be performed before the judgment asto whether to change the SeNB or may be performed together with thisjudgment.

The T-MeNB does not change the SeNB, and thus, performs admissioncontrol due to a change of the MeNB. In admission control, the T-MeNBperforms an RRC-related configuration for T-MeNB. Examples of theRRC-related configuration include an AS configuration indicating theconfiguration of a radio resource and an RRM configuration indicatingRRM information. The T-MeNB may configure the information included inthe RRC connection reconfiguration message.

A UE-dedicated RACH preamble configuration used by the T-MeNB may beprovided and configured separately from a UE-dedicated RACH preambleconfiguration used by the SeNB. Also, a cell-radio network temporaryidentifier (C-RNTI) used by the T-MeNB may be provided and configuredseparately from the C-RNTI used by the SeNB. In the case where, forexample, the SeNB supports another UE not being served by the T-MeNB,the configuration and C-RNTI can be configured dedicatedly to each UE,thus enabling flexible control.

Alternatively, a UE-dedicated RACH preamble configuration used by theT-MeNB may be configured to be identical to a UE-dedicated RACH preambleconfiguration used by the SeNB. Also, the C-RNTI used by the T-MeNB maybe configured to be identical to the C-RNTI used by the SeNB. In thecase where, for example, the SeNB does not support another UE not beingserved by the T-MeNB, the configuration and C-RNTI need not to beconfigured dedicatedly to each UE and can be controlled by one value,resulting in simple control.

The RRC-related configuration for T-MeNB needs the information forallowing the UE to perform an RRC connection reconfiguration with theT-MeNB.

When deciding to accept a HO request, in step ST1104, the T-MeNBnotifies the S-MeNB of a handover request Ack message including a HOcommand message. This HO command message may include the configurationinformation related to RRC for T-MeNB described above.

When keeping the bearer 2 using the SeNB, the T-MeNB may notify theS-MeNB that HO is enabled while keeping the configuration of the bearer2 using the SeNB. When keeping the configuration of the bearer 2 usingthe SeNB, the T-MeNB may notify the S-MeNB that HO is enabled whilekeeping the RRC-related configuration for SeNB. These may be notified ina handover request Ack message, may be notified together with a handoverrequest Ack message to the bearer not using the SeNB, or may be notifiedseparately from a handover request Ack message to the bearer not usingthe SeNB.

The handover request Ack message or the HO command message may includethe information indicating whether there is a change in the RRC-relatedconfiguration of the SeNB. When obtaining the information andrecognizing that there is a change, the S-MeNB may further obtain theRRC-related configuration of the SeNB. This simplifies the process whenthere is no change.

When there is no change in the RRC-related configuration of the SeNB,the configuration information needs not to be included in a HO commandmessage. This reduces an amount of information to be signaled.

Alternatively, the HO command message may include the configurationinformation related to RRC for SeNB. The S-MeNB can compare theconfiguration information related to RRC for SeNB obtained from theT-MeNB with the configuration information related to RRC for SeNB of itsown cell, thus reliably verifying that there is no change.

The S-MeNB that has received the handover request Ack message in stepST1104 can recognize that the relevant HO is HO to the T-MeNB whilekeeping the SeNB to the UE during dual connectivity.

The S-MeNB which has recognized that it is to perform HO while keepingthe SeNB does not request the SeNB to change the configuration of thebearer 2. The S-MeNB does not request to change the RRC-relatedconfiguration for SeNB.

In step ST1105, the process of notifying the SeNB of the informationindicating that HO is to be activated is performed among the T-MeNB, theS-MeNB, and the SeNB. For example, the T-MeNB notifies the SeNB of theinformation indicating that HO is to be activated. This notification maybe performed when the T-MeNB transmits a handover request Ack message tothe S-MeNB. Alternatively, the S-MeNB that has received the handoverrequest Ack message from the T-MeNB may notify the SeNB of theinformation indicating that HO is to be activated.

In step ST1108 of FIG. 18, the S-MeNB forwards the PDCP SN informationand the yet-to-be-transmitted data to the T-MeNB and establishesloss-free communication.

A change of the configuration of the bearer 2 activated by the SeNB maybe avoided while the UE during dual connectivity is performing HObetween MeNBs. Any change in the configuration of the bearer activatedby the SeNB during HO can trigger the process when the MeNB is changedfrom the S-MeNB to the T-MeNB, leading to complicated control, whichincreases a possibility of malfunction. A change of the configuration ofthe bearer 2 activated by the SeNB during HO is avoided, thus reducingmalfunctions.

Disclosed below is a method of avoiding a change of the configuration ofthe bearer 2 activated by the SeNB during HO.

The T-MeNB notifies the SeNB of the information indicating that HO is tobe activated. This notification may be performed when the T-MeNBtransmits a handover request Ack message to the S-MeNB. Alternatively,the S-MeNB that has received the handover request Ack message from theT-MeNB may notify the SeNB of the information indicating that HO is tobe activated.

The SeNB that has received the information indicating that HO has beenactivated does not activate a request to change the configuration of thebearer 2. The SeNB may avoid activating the request until the HOcompletes.

In another method, during the HO, the S-MeNB or the T-MeNB may notifythe SeNB of Nack (or rejection) in response to the request to change theconfiguration of the bearer 2 from the SeNB. Cause information may beincluded, and the information indicating that Nack (or rejection) isnotified because it is during HO may be provided and configured as acause.

The method in which a SeNB recognizes the completion of HO will bedisclosed below. The T-MeNB may notify the SeNB of the informationindicating the completion of HO. This notification may be made when theT-MeNB receives a path switch request Ack message from the MME in the HOprocess. Alternatively, the completion of HO may be the completion of aMeNB change process described below.

This prevents, in the HO process of a UE during dual connectivity, achange of the configuration of the bearer 2 to the UE activated by theSeNB. Malfunctions are thus reduced as a system.

Another method will be disclosed. Although it has been disclosed thatwhen performing HO while keeping the SeNB, the S-MeNB does not requestthe SeNB to change the configuration of the bearer 2 and to change theRRC-related configuration for the SeNB, in another method, whenperforming HO while keeping the SeNB, the S-MeNB may notify the SeNBthat the relevant HO is HO while keeping the SeNB. Alternatively, theS-MeNB may notify that it does not request the SeNB to change the splitbearer configuration and to change the RRC-related configuration forSeNB. This may be notified together with the information indicating thatHO is to be activated.

This allows the SeNB to recognize during HO, for example, whether therelevant HO is HO to be performed while keeping the SeNB. For HO whilekeeping the SeNB, during the HO, the SeNB can avoid the change of theconfiguration of the bearer 2 to the UE activated by the SeNB. Similareffects described above can be achieved.

In step ST1106, the S-MeNB that has received the handover request Ackmessage notifies the UE of an RRC connection reconfiguration messageincluding mobility control information (abbreviated as MCI) to cause theUE to perform HO to the T-MeNB. The S-MeNB that has received thehandover request Ack message may notify the UE that there is no changein the bearer using the SeNB.

The S-MeNB notifies the UE that there is no change in the SeNB for E-RABthat configures the bearer 2 during the HO. Or, the S-MeNB may notifythat there is no change in the RRC-related configuration for SeNB. Anotification may be made by being included in an RRC connectionreconfiguration message including the MCI.

When recognizing the configuration information related to RRC for theSeNB, the S-MeNB may notify the UE of the configuration information asdesired. The UE can compare the configuration information related to RRCfor the SeNB connected with its own UE and the received configurationinformation, thus reliably verifying that there is no change.

The UE that has received the RRC connection reconfiguration messageincluding MCI in step ST1106 performs HO from the S-MeNB to the T-MeNBwhile remaining synchronized and connected with the SeNB. The UEperforms HO while keeping a radio resource with the SeNB. In this state,the UE can communicate with the SeNB.

The method of handling uplink data from the UE to the SeNB will bedisclosed below. When performing HO while remaining connected with theSeNB, the UE can keep transmitting the uplink data of the bearer 2. Or,the UE can stop transmitting the uplink data and store the uplink datain the buffer during HO until the completion of HO from the S-MeNB tothe T-MeNB. This provides a time interval before the establishment of acontrol plane, thus reducing, for example, a process of buffering thecontrol information between the SeNB and the S-MeNB or between the SeNBand the T-MeNB when the control plane path is yet to be established,which simplifies the process of the SeNB.

In step ST1107, the UE that has received, from the S-MeNB, the MCIinstructing HO to the T-MeNB in the step ST1106 stops transmitting theuplink data of the bearer 1 and stores the uplink data in the buffer.

In step ST1109, the UE performs a process of changing connection fromthe S-MeNB to the T-MeNB in accordance with the MCI. After receiving theMCI, the UE disconnects the connection with the S-MeNB and is thenconnected with the T-MeNB. The UE reconfigures the RRC connection forthe T-MeNB using the contents received in the RRC connectionreconfiguration message in step ST1106, thereby being connected with theT-MeNB.

The UE performs an RA procedure with the T-MeNB in step ST1110 andnotifies the T-MeNB of an RRC connection reconfiguration completemessage in step ST1111.

After notifying the T-MeNB of the RRC connection reconfigurationcomplete message, the UE can perform data communication directly withthe T-MeNB.

After notifying of the RRC connection reconfiguration complete messagewith the T-MeNB, the UE transmits uplink data of the bearer 1.

The method of handling downlink data to be notified to the UE by theSeNB will be disclosed below. When performing HO while remainingconnected with the UE, the SeNB can keep transmitting the downlink dataof the bearer 2.

Alternatively, the SeNB can also stop transmitting the downlink data andstore the downlink data in the buffer during HO until the completion ofHO from the S-MeNB to the T-MeNB. This provides a time interval beforethe establishment of a control plane, thus reducing, for example, aprocess of buffering the control information between the SeNB and theS-MeNB or between the SeNB and the T-MeNB when the control plane path isyet to be established, which simplifies the process of the SeNB.

The SeNB is notified of the configuration of a bearer for dualconnectivity to the UE by the MeNB. The configuration informationrelated to RRC of the SeNB to the UE is notified by the MeNB.

The SeNB thus needs to recognize, for example, by which MeNB theconfiguration of the bearer has been notified, and to which MeNB theSeNB should notify of the configuration information related to RRC thathas been configured by the SeNB. In other words, the SeNB needs torecognize the MeNB.

The SeNB performs data communication for the UE that performs dualconnectivity between MeNBs. The SeNB needs to recognize data from whichMeNB the SeNB should transmit to the UE, and to which MeNB the SeNBshould transmit the data from the UE. In other words, the SeNB needs torecognize the MeNB.

Although the former MeNB and the latter MeNB may be configuredseparately, a case where they are the same will now be described. In thedescription below, the MeNB is referred to as a control MeNB for theSeNB.

Disclosed below is a method of recognizing, in the HO process by the UEduring dual connectivity, to which MeNB the control MeNB for the SeNBhas been changed in the case where the SeNB is not changed and the MeNBis changed from the S-MeNB to the T-MeNB. The following two, (1) and(2), will be disclosed as specific examples.

The following two, (1) and (2), will be disclosed as specific examplesof the method of recognizing to which MeNB the control MeNB for the SeNBhas been changed.

(1) The S-MeNB notifies the SeNB of a change of the control MeNB.

(2) The T-MeNB notifies the SeNB of a change of the control MeNB.

The specific example (1) will be disclosed below specifically. Uponreceipt of an RRC connection reconfiguration complete message from theUE, the T-MeNB notifies the S-MeNB of a message requesting to change thecontrol MeNB for the SeNB so as to request the SeNB to change a controlMeNB. The notification may be made through X2 signaling. The identifierof a SeNB that is to change the control MeNB may be included in thisnotification to be notified. The change cause information may beincluded to be notified. Or, the information indicating that a change ofthe MeNB is due to HO may be provided to be notified.

The S-MeNB that has received the notification notifies the SeNB that isto change a control MeNB of a control MeNB change request messagerequesting to change a control MeNB. This notification may be madethrough X2 signaling. Alternatively, this notification may be madethrough signaling on an interface provided between the MeNB and theSeNB.

The following five, (1) to (5), will be disclosed as specific examplesof the information included in the signaling.

(1) Information indicating a change of the control MeNB.

(2) Identifier of a control MeNB after change, which is herein theidentifier of a T-MeNB.

(3) Bearer identifier of a path using a SeNB, which is herein an E-RABidentifier (which may be, for example, E-RAB ID) or an EPS beareridentifier.

(4) Identifier of a UE that performs dual connectivity using a bearer.

(5) Combination of (1) to (4) above.

The SeNB that has received the control MeNB change request messagespecifies a bearer whose MeNB is to be changed from the informationincluded in the control MeNB change request message. The SeNB changesthe control MeNB of the bearer to a MeNB after change. The SeNB maymanage the bearer in association with the identifier of the controlMeNB. In other words, the SeNB may associate the identifier of thebearer with the identifier of the control MeNB. This allows the SeNB tochange a control MeNB of a bearer for the UE that performs dualconnectivity.

After changing a control MeNB, the SeNB accepts a modification of thebearer and a request for release only from the control MeNB afterchange. Additionally, the SeNB performs data communication with the MeNBafter change.

This allows the SeNB to recognize to which MeNB the control MeNB hasbeen changed and to perform, between the control MeNB after change anditself, communication for control of, for example, a request to modifyand release the bearer to the UE and data communication.

The SeNB that has changed the control MeNB may notify the S-MeNB of acontrol MeNB change response message. This message may include theinformation indicating that the control MeNB has been changed.

The S-MeNB that has received the control MeNB change response messagefrom the SeNB may notify the control MeNB after change, here, the T-MeNBof a message indicating that a change of the control MeNB for the SeNBhas completed.

This allows the MeNB after change, here, the T-MeNB to recognize thatthe control MeNB for the SeNB has been changed and to start, to theSeNB, communication for control of, for example, a request to modify andrelease the bearer to the UE and data communication.

The MeNB after change may notify the SeNB of a message confirming thatthe control MeNB of the bearer to the UE has been changed. The SeNB mayrespond to the message.

The specific example (2) will be disclosed below specifically. Uponreceipt of the RRC connection reconfiguration complete message from theUE, the T-MeNB notifies the SeNB of a control MeNB change requestmessage. Used as the identifier of the SeNB may be SeNB informationincluded in the HO request message received from the S-MeNB. Thisnotification may be made through X2 signaling or through signaling onthe interface provided between the MeNB and the SeNB. The informationdisclosed in the method of the specific example (1) is applicable as theinformation included in the signaling.

The process by the SeNB that has received the control MeNB changerequest message from the T-MeNB is similar to that of the specificexample (1), and thus, description thereof will be omitted.

This allows the SeNB to recognize to which MeNB the control MeNB hasbeen changed and to perform, between the control MeNB after change anditself, communication for control of, for example, a request to modifyand release the bearer to the UE and data communication.

The SeNB that has changed the control MeNB may notify the T-MeNB of acontrol MeNB change response message. This message may include theinformation indicating that the control MeNB has been changed.

The T-MeNB that has received the control MeNB change response messagefrom the SeNB may notify the control MeNB before change, here, theS-MeNB of a message indicating that a change of the control MeNB for theSeNB has completed.

This allows the MeNB before change, here, the S-MeNB to recognize thatthe control MeNB for the SeNB has been changed and to end thecommunication for control of, for example, a request to modify andrelease the bearer to the UE and data communication to the SeNB.

When UE during dual connectivity using a SeNB has performed HO, the SeNBcan perform communication for control related to the bearerconfiguration and data communication with the MeNB after change(T-MeNB).

The UE and the T-MeNB can thus perform data communication via the SeNB.

A specific example of the process of changing a MeNB for the SeNB willbe described. In step ST1112, the T-MeNB that has received the RRCconnection reconfiguration complete message from the UE in step ST1111notifies the S-MeNB of a control MeNB change request message requestingto change the control MeNB for the SeNB. The identifier of the SeNB thatis to change the control MeNB is included in the message requesting tochange the control MeNB for the SeNB.

In step ST1113, the S-MeNB that has received the message notifies theSeNB that is to change the control MeNB of a control MeNB change requestmessage. The control MeNB change request message includes theinformation indicating a change of the control MeNB, the identifier ofthe T-MeNB, the bearer identifier of the path using the SeNB, theidentifier of a bearer whose control MeNB is to be changed in the SeNB,the identifier of the S-MeNB, and the identifier of a UE being a HOtarget that is to perform dual connectivity using the SeNB. This allowsthe SeNB to recognize for which bearer configured by which MeNB the SeNBshould change a control MeNB.

In step ST1114, the SeNB that has received the control MeNB changerequest message uses the received information to change a control MeNB.This allows the SeNB to perform communication for control related to abearer and data communication with the T-MeNB that is a control MeNBafter change. The SeNB that has changed the control MeNB ends thecommunication for control and data communication with the control MeNBbefore change.

In step ST1115, the SeNB that has changed the control MeNB in stepST1114 notifies the S-MeNB of a control MeNB change response message fornotifying that the SeNB has changed the control MeNB. In step ST1116,the S-MeNB that has received the control MeNB change response messagenotifies the T-MeNB of a message responding to the change of the controlMeNB for the SeNB in order to notify that a change of the control MeNBfor the SeNB has completed. This allows the T-MeNB to recognize that thecontrol MeNB for the SeNB has been changed to the T-MeNB. The T-MeNB canthus perform communication for control related to a bearer and datacommunication with the SeNB.

In this embodiment, as described above, the SeNB is notified of thecontrol MeNB change request message via the S-MeNB. Alternatively,without via the S-MeNB, a control MeNB change request message and acontrol MeNB change response message may be transmitted and receiveddirectly between the T-MeNB and the SeNB. In that case, the informationabout a S-MeNB may be included in a control MeNB change request messageand used to determine that the request is not an illegal control MeNBchange request in the process of changing a control MeNB for the SeNB.

Steps ST1112 to ST1116 shown in FIG. 18 are referred to as step ST1117.Step ST1117 represents the process of changing a control MeNB for theSeNB that is performed among the T-MeNB, the S-MeNB, and the SeNB.

The path switch process by the S-GW for changing a MeNB is similar tothat of FIG. 12 described above, and thus, description thereof will beomitted. The path switch process enables data communication via theT-MeNB between the S-GW and the UE being a HO target.

As to the data communication between the S-GW and the UE, through onepath, data is communicated directly between the S-MeNB and the UE, whilethrough the other path, data is communicated directly between the SeNBand the UE. The method disclosed in this embodiment allows the UE duringdual connectivity using a bearer to perform HO between MeNBs.

Another method of the process of changing a control MeNB for the SeNBwill be disclosed. In the method disclosed above, the process ofchanging a control MeNB for the SeNB is performed after the T-MeNBreceives an RRC connection reconfiguration complete message from the UE.

In another method, the process of changing a control MeNB for the SeNBmay be performed after the T-MeNB notifies the S-MeNB of a handoverrequest Ack message, after the S-MeNB receives a handover request Ackmessage from the T-MeNB, or after the S-MeNB transmits an RRC connectionreconfiguration message including MCI to the UE. The method describedabove may be applied to the process of changing a control MeNB for theSeNB.

Consequently, a control MeNB can be changed early to the T-MeNB by theSeNB, allowing control data communication to be performed early betweenthe SeNB and the T-MeNB.

Another method of the process of changing a control MeNB for the SeNBwill be disclosed. The process of changing a control MeNB for the SeNBmay be performed after the T-MeNB completes a path switch process forthe MME and the S-GW. Upon receipt of a path switch request Ack messagefrom the MME, the T-MeNB notifies the SeNB that the MeNB is to bechanged.

Alternatively, upon receipt of a UE context release message from theT-MeNB, the S-MeNB may notify the SeNB that the MeNB is to be changed.The method described above may be applied to the process of changing acontrol MeNB for the SeNB.

Consequently, data communication by a bearer using a SeNB can beperformed after the HO process, which includes the path switch of theMIME and the S-GW to the T-MeNB, completes. Data communication isperformed after the reliable completion of the HO process, thuspreventing the control of data communication from becoming morecomplicated.

According to this embodiment, as described above, upon activation of theHO process, the SeNB being a small cell is notified that a macro cellwhich controls the small cell is to be changed. This enables theexecution of the HO process while holding a path through whichcommunication is performed from the S-GW via the small cell, thusallowing the user plane data communication to be continued even duringthe HO process.

Compared with the case where HO is executed after releasing the SeNB asin the first embodiment, the control sequence is simplified, thusreducing an amount of information to be signaled in the HO sequencestarting from during the dual connectivity.

Third Embodiment

FIGS. 20 to 22 show an example sequence of a handover-related process ina communication system of a third embodiment of the present invention.FIG. 20 is continuous with FIG. 21 at a boundary BL3. FIG. 21 iscontinuous with FIG. 22 at a boundary BL4. The handover-related processof this embodiment is similar to the handover-related processes of thefirst embodiment shown in FIGS. 10 to 13 and the second embodiment shownin FIGS. 17 to 19 described above, and thus, the same steps will bedenoted by the same steps numbers and description thereof will beomitted.

This embodiment will disclose a method of performing HO withoutreleasing a SeNB in the case of Alternative 3C of the user planearchitecture of dual connectivity described in Non-Patent Document 11(see 8.1.1.8 of Non-Patent Document 11).

In Alternative 3C of the user plane architecture, the MeNB performsbearer split in which a bearer is split into a path through which theMeNB and the UE directly communicate with each other and a path throughwhich the MeNB and the UE communicate with each other via a small cell.

When the UE during dual connectivity by bearer split performs HO betweenMeNBs, the connection with the SeNB is not released. In other words, HObetween MeNBs is performed while keeping dual connectivity of the UE.

Although description has been made also in the first modification of thefirst embodiment, in bearer split, the E-RAB corresponding to one EPSbearer is separated into two paths by the S-MeNB, and data iscommunicated between the S-GW and the UE. Through one path, data iscommunicated directly between the S-MeNB and the UE in step ST1001.Through the other path, data is communicated between the S-MeNB and theUE via the SeNB in steps ST1003 and ST1004. In step ST1002, data iscommunicated between the S-MeNB and the S-GW through one path.

Described below is a case where the UE during dual connectivity performsHO between MeNBs by bearer split.

The method disclosed in the first embodiment is applicable as the methodin which the MeNB notifies the UE during dual connectivity by bearersplit of a measurement control message. Description thereof will beomitted here.

Described below is a case where the reception quality of an S-MeNB hasdegraded and the reception quality of a T-MeNB has improved in themeasurement, and a measurement report is performed in accordance withevent criteria.

In step ST906, the UE performs a measurement report to the S-MeNB. Themeasurement report from the UE may include the identifier and thereception quality of the SeNB used in dual connectivity.

The S-MeNB that has received the measurement report from the UE decidesto cause the UE to perform HO to the T-MeNB using the result of thereport.

In step ST1201, the S-MeNB notifies the T-MeNB of a HO request message.The SeNB information may be included in this message to be notified. Thebearer information related to a bearer being subjected to bearer splitusing the SeNB may be notified. The information related to bearer splitmay be notified. Or, the RRC-related information in bearer split may benotified.

Examples of the SeNB information include the identifier of the SeNB andthe reception quality of the SeNB by the UE.

A non-limiting example of the bearer information related to a bearerbeing subjected to bearer split is an E-RAB identifier.

A non-limiting example of the information related to bearer split is theinformation related to the configuration (hereinafter also referred toas a “split bearer configuration”) of a bearer (hereinafter alsoreferred to as a “split bearer”) split into a direct path between theMeNB and the UE and a path between the MeNB and the UE via the SeNB. TheMeNB may configure a split bearer for each split path. The identifiermay be provided corresponding to each path. The identifier of each pathand the information related to the split bearer configuration that isconfigured for each path may be notified in association with each other.For the path using a SeNB, the identifier of the SeNB and the identifierof the path may be notified in association with each other. Examples ofthe information related to a split bearer configuration that isconfigured for each path include QoS parameters such as QCI, ARP, andguaranteed bit rate QoS information.

This enables a split bearer configuration per path, thus allowing theS-MeNB to notify the T-MeNB of the split bearer configuration per path.The T-MeNB can obtain the split bearer configuration per path anddetermine whether the split bearer configuration per path needs to bechanged.

The bearer information related to a bearer subjected to bearer split andthe information related to a split bearer configuration may be notifiedin association with each other. The bearers subjected to and notsubjected to bearer split are distinguishable from each other.

A non-limiting example of the configuration information related to RRCin bearer split is an RRC context. Examples of the RRC context includean AS configuration indicating the configuration of a radio resource andan RRM configuration indicating the RRM information.

The RRC-related information includes the RRC-related information forMeNB and the RRC-related information for SeNB. Each piece of RRC-relatedinformation may be discernable. For example, an RRC context for MeNB andan RRC context for SeNB are created separately. Alternatively, theinformation for MeNB and the information for SeNB may be createdseparately in one RRC context.

Consequently, the RRC-related information for SeNB and the RRC-relatedinformation for MeNB are configurable dedicatedly, so that the S-MeNBcan notify the T-MeNB of the RRC-related information of each of the MeNBand the SeNB. The T-MeNB can obtain the RRC-related information of eachof the S-MeNB and the SeNB. The T-MeNB can thus determine whether theRRC-related configuration of the SeNB needs to be changed.

In step ST1101, the T-MeNB uses the information received in the HOrequest message from the S-MeNB to determine whether to change the SeNB.The process of step ST1101 is similar to the process of step ST1101 ofFIG. 17, and thus, description thereof will be omitted.

If judging to change the SeNB, the T-MeNB moves to step ST1102. Themethod disclosed in the second embodiment may be applied to the processof step ST1102, description of which will be omitted here.

If judging not to change the SeNB, in step ST1103 the T-MeNB decides toperform the HO process while continuously using the SeNB. In otherwords, the T-MeNB decides to change only the MeNB. If judging not tochange the SeNB, the T-MeNB keeps the split bearer configuration usingthe SeNB. The T-MeNB does not perform the process of changing the splitbearer configuration to the SeNB.

In step ST1202, the T-MeNB performs admission control in response to aHO request. The admission control may be performed before the judgmentas to whether to change the SeNB or may be performed together with thisjudgment.

The T-MeNB does not change the SeNB, and thus, performs admissioncontrol due to a change of the MeNB. In admission control, the T-MeNBperforms an RRC-related configuration for T-MeNB. Examples of theRRC-related configuration include an AS configuration indicating theconfiguration of a radio resource and an RRM configuration indicatingRRM information. The T-MeNB may configure the information included inthe RRC connection reconfiguration message.

The UE-dedicated RACH preamble configuration used by the T-MeNB may beprovided and configured separately from the UE-dedicated RACH preambleconfiguration used by the SeNB. The C-RNTI used by the T-MeNB may beprovided and configured separately from the C-RNTI used by the SeNB. Inthe case where, for example, the SeNB supports another UE not beingserved by the T-MeNB, the configuration or C-RNTI can be configureddedicatedly to each UE, thus enabling flexible control.

Alternatively, the UE-dedicated RACH preamble configuration used by theT-MeNB may be configured to be identical to the UE-dedicated RACHpreamble configuration used by the SeNB. Also, the C-RNTI used by theT-MeNB may be configured to be identical to the C-RNTI used by the SeNB.In the case where, for example, the SeNB does not support another UE notbeing served by the T-MeNB the configuration and C-RNTI need not to beconfigured dedicatedly to each UE and can be controlled by one value,resulting in simple control.

The RRC-related configuration for T-MeNB needs the information thatenables the RRC connection reconfiguration of the UE with the T-MeNB.

When deciding to accept a HO request, in step ST1203, the T-MeNBnotifies the S-MeNB of a handover request Ack message including a HOcommand message. This HO command message may include the configurationinformation related to RRC for T-MeNB described above.

When keeping the configuration of a split bearer using the SeNB, theT-MeNB notifies the S-MeNB of the identifier of the SeNB, the E-RABidentifier, the information related to bearer split, the RRC-relatedinformation in bearer split, and the like. The T-MeNB may notify theS-MeNB that HO is enabled while keeping the configuration of a splitbearer using the SeNB. Or, when keeping the configuration of a splitbearer using the SeNB, the T-MeNB may notify the S-MeNB that HO isenabled while keeping the RRC-related configuration for SeNB.

This reduces an amount of information to be notified. Such anotification may be performed in a handover request Ack message, may beperformed together with a handover request Ack message to a bearer notusing the SeNB, or may be performed separately from a handover requestAck message to a bearer not using the SeNB.

The handover request Ack message or the HO command message may includethe information indicating whether there is a change in the RRC-relatedconfiguration of the SeNB. When obtaining the information andrecognizing that there is a change, the S-MeNB may further obtain theRRC-related configuration of the SeNB. This simplifies the process whenthere is no change.

When there is no change in the RRC-related configuration of the SeNB,the configuration information needs not to be included in a HO commandmessage. This reduces an amount of information to be signaled.

Alternatively, the HO command message may include the configurationinformation related to RRC for SeNB. The S-MeNB can compare theconfiguration information related to RRC for SeNB obtained from theT-MeNB with the configuration information related to RRC for SeNB of itsown cell, thus reliably verifying that there is no change.

The S-MeNB that has received the handover request Ack message in stepST1203 can recognize that the relevant HO is HO to the T-MeNB whilekeeping the SeNB to the UE during dual connectivity.

The S-MeNB which has recognized that it is to perform HO while keepingthe SeNB does not request the SeNB to change the split bearerconfiguration. The S-MeNB does not request to change the RRC-relatedconfiguration for SeNB.

A change of the bearer split configuration activated by the SeNB may beavoided while the UE during dual connectivity is performing HO betweenMeNBs. Any change in the bearer split configuration activated by theSeNB during HO can trigger the process when the MeNB is changed from theS-MeNB to the T-MeNB, leading to complicated control, which increases apossibility of malfunction. A change of the bearer split configurationactivated by the SeNB during HO is avoided, thus reducing malfunctions.

Disclosed below is a method of avoiding a change of the bearer splitconfiguration activated by the SeNB during HO.

The T-MeNB notifies the SeNB of the information indicating that HO is tobe activated. This notification may be performed when the T-MeNBtransmits a handover request Ack message to the S-MeNB. Alternatively,the S-MeNB that has received a handover request Ack message from theT-MeNB may notify the SeNB of the information indicating that HO is tobe activated.

The SeNB that has received the information indicating that HO has beenactivated does not activate a request to change the bearer splitinformation. The SeNB may avoid activating the request until the HOcompletes.

In another method, during the HO, the S-MeNB or the T-MeNB may notifythe SeNB of Nack (or rejection) in response to the bearer splitconfiguration change request from the SeNB. Cause information may beincluded, and the information indicating that Nack (or rejection) isnotified because it is during HO may be provided and configured as acause.

In the processes of FIGS. 20 to 22, in step ST1204, these processes areperformed among the T-MeNB, the S-MeNB, and the SeNB.

The method in which a SeNB recognizes the completion of HO will bedisclosed below. The T-MeNB may notify the SeNB of the informationindicating the completion of HO. This notification may be performed whenthe T-MeNB receives a path switch request Ack message from the MME inthe HO process. Alternatively, the completion of HO may be thecompletion of a process of changing a MeNB described below.

This prevents, in the HO process of a UE during dual connectivity, achange of the bearer split configuration to the UE activated by theSeNB. Malfunctions are thus reduced as a system.

Another method will be disclosed. Although it has been disclosed thatwhen performing HO while keeping the SeNB as described above, the S-MeNBdoes not request the SeNB to change the split bearer configuration andto change the RRC-related configuration for SeNB, in another method,when performing HO while keeping the SeNB, the S-MeNB may notify theSeNB that the relevant HO is HO while keeping the SeNB. Alternatively,the S-MeNB may notify that it does not request the SeNB to change thesplit bearer configuration and to change the RRC-related configurationfor SeNB. This may be notified together with the information indicatingthat HO is to be activated.

This allows the SeNB to recognize during HO, for example, whether therelevant HO is HO to be performed while keeping the SeNB. In the case ofHO while keeping the SeNB, the SeNB can avoid the change of the bearersplit configuration to the UE activated by the SeNB during the HO.Effects similar to those described above can be achieved.

In step ST1205, the S-MeNB that has received the handover request Ackmessage notifies the UE of an RRC connection reconfiguration messageincluding mobility control information (MCI) to cause the UE to performHO to the T-MeNB. The S-MeNB that has received the handover request Ackmessage may notify the UE that there is no change in the bearer splitusing the SeNB.

The S-MeNB notifies the UE that there is no change in the SeNB for E-RABthat configures a split bearer. The S-MeNB may notify that there is nochange in the RRC-related configuration for SeNB. A notification may bemade by being included in the RRC connection reconfiguration messageincluding the MCI.

When recognizing the configuration information related to RRC for SeNB,the S-MeNB may notify the UE of the configuration information asdesired. The UE can compare the configuration information related to RRCto the SeNB connected with its own UE and the received configurationinformation, thus reliably verifying that there is no change.

The UE that has received the RRC connection reconfiguration messageincluding MCI in step ST1205 performs HO from the S-MeNB to the T-MeNBwhile remaining synchronized and connected with the SeNB. The UEperforms HO while keeping the radio resource with the SeNB. For example,the UE does not reset the MAC configuration to the SeNB. Alternatively,the UE does not reconfigure the RLC configuration to the SeNB. In thisstate, the UE can communicate with the SeNB.

The configuration to the MeNB conforms to the configuration informationrelated to RRC to the MeNB in the RRC connection reconfiguration messageincluding the MCI. The PDCP is located at the MeNB, and thus, theconfiguration of the PDCP may conform to the configuration to the MeNB.For example, a reconfiguration is made from the configuration to theS-MeNB to the configuration to the T-MeNB. In the RRC connectionreconfiguration message including MCI, the configuration informationrelated to RRC to the MeNB and the configuration information related toRRC to the SeNB may be provided individually so as to be configurableindividually. This allows the UE to individually reconfigure or keep theconfiguration to the SeNB and the configuration to the MeNB, or to shiftthese configurations to default configurations. In this example, the UEcan reconfigure the configuration to the MeNB while keeping theconfiguration to the SeNB, and thus can change the MeNB from the S-MeNBto the T-MeNB while keeping the connection with the SeNB.

Disclosed below is a method of handing uplink data from the UE to theSeNB. Even when performing HO while remaining connected with the SeNB,the UE does not recognize whether the SeNB is connected with the S-MeNBor the T-MeNB. During the HO process, if the UE transmits uplink data tothe SeNB when the SeNB is yet to be connected to the T-MeNB, the SeNBcannot transmit the uplink data to the T-MeNB, resulting in a loss ofthe uplink data. A method of solving such a problem will now bedisclosed.

After receiving MCI from the S-MeNB, the UE stops transmitting uplinkdata and stores the uplink data in the buffer. The UE also stopstransmitting uplink data until the RRC connection reconfiguration withthe T-MeNB completes, and stores the uplink data in the buffer. The UEtransmits the uplink data after notifying of an RRC connectionreconfiguration complete message with the T-MeNB. The UE may stoptransmitting new uplink data after receiving MCI from the S-MeNB andstore the uplink data in the buffer. The uplink data may be all piecesof uplink data transmitted from the UE. The UE may stop transmitting theuplink data before the division into uplink data to be transmittedthrough a direct path to the T-MeNB and uplink data to be transmittedthrough a path to the T-MeNB via the SeNB, and store them in the buffer.Alternatively, if the uplink data to be transmitted through the directpath to the T-MeNB and the uplink data to be transmitted through thepath to the T-MeNB via the SeNB are divided within the UE, the UE maystop transmitting these pieces of uplink data and store these pieces ofuplink data in the buffer. Numbering of PDCP SNs may be performed as theprocess for uplink data by the UE. This solves a problem of anoccurrence of uplink data loss.

In step ST1206, the UE that has received MCI instructing HO to theT-MeNB from the S-MeNB in step ST1205 stops transmitting the uplink dataand stores the uplink data in the buffer. In step ST1207, the UEperforms a process of changing connection from the S-MeNB to the T-MeNBin accordance with the MCI.

In step ST1207, the UE receives the MCI, and then, disconnects theconnection with the S-MeNB to be connected to the T-MeNB. The UEreconfigures RRC connection for the T-MeNB using the contents receivedin the RRC connection reconfiguration message of step ST1205, to therebybeing connected to the T-MeNB.

The UE performs the RA procedure with the T-MeNB in step ST1212 andnotifies the T-MeNB of an RRC connection reconfiguration completemessage in step ST1213.

The UE can perform data communication directly with the T-MeNB afternotifying the T-MeNB of the RRC connection reconfiguration completemessage.

The UE transmits pieces of uplink data after notifying of the RRCconnection reconfiguration complete message with the T-MeNB. The piecesof uplink data are divided by the UE for one path through which uplinkdata is directly transmitted to the T-MeNB and the other path throughwhich uplink data is transmitted to the T-MeNB via the SeNB, and the UEtransmits the pieces of uplink data individually to the T-MeNB and theSeNB.

In step ST1219, the UE transmits uplink data through the path throughwhich uplink data is directly transmitted to the T-MeNB. In step ST1220,the UE transmits uplink data to the SeNB. The uplink data may betransmitted in accordance with the PDCP SN.

The UE or the SeNB may perform the synchronization process between theUE and the SeNB while the UE is performing HO from the S-MeNB to theT-MeNB. Alternatively, the UE may perform the synchronization processbefore starting transmitting uplink data to the SeNB. The UE may use theRA procedure.

The UE may continue the transmission process for the uplink data to theS-MeNB, which has been transmitted and received to and from the SeNBbefore receiving MCI, or, before stopping transmitting uplink data. TheUE may also continue the transmission process for uplink data, which hasbeen directly transmitted and received to and from the S-MeNB, until nouplink data is delivered. These may be performed using, for example, anoverhead compression configuration notified by the S-MeNB.

The UE may start transmitting, to the T-MeNB, the pieces of uplink datatransmitted and received to and from the S-MeNB directly or via the SeNBbefore receiving MCI, or, before stopping transmitting uplink datastarting from data whose transmission has failed. The UE may starttransmitting data to the T-MeNB, starting from the data with thesmallest PDCP SN among the pieces of data whose transmissions havefailed. The UE may transmit the pieces of uplink data to the T-MeNBdirectly or via the SeNB, starting from the relevant data.

This can minimize a data loss. This method requires few buffers foruplink data in the SeNB, reducing the buffer capacity of the SeNB andsimplifying the configuration thereof The SeNB can be achievedinexpensively.

As to the transmission to the T-MeNB, the header compressionconfiguration configured by the T-MeNB may be used in both of thetransmission directly to the T-MeNB and the transmission to the T-MeNBvia the SeNB.

In some cases, the SeNB is yet to be connected with the T-MeNB in thepath through which uplink data is transmitted to the T-MeNB via theSeNB. In such a case, the SeNB may store the uplink data from the UE inthe buffer until a change of the MeNB configuration with the T-MeNBcompletes. The SeNB transmits the uplink data from the UE to the T-MeNBafter the change of the MeNB configuration with the T-MeNB completes.The SeNB may reorder the pieces of uplink data using the PDCP SNsthereof and transmit the reordered pieces of uplink data.

This solves the problem of an occurrence of data loss in the SeNB evenwhen a change of the MeNB configuration delays for some reason betweenthe SeNB and the T-MeNB, so that uplink data is transmitted from the UEto the SeNB.

Another method of solving the problem of an occurrence of uplink dataloss will be disclosed. The UE transmits uplink data to the SeNB withoutwaiting for the completion of the connection with the T-MeNB. The SeNBstores the uplink data from the UE in the buffer until the change of theMeNB configuration with the T-MeNB completes. The SeNB transmits theuplink data from the UE to the T-MeNB after the change of the MeNBconfiguration with the T-MeNB completes. The SeNB may reorder the piecesof uplink data using the PDCP SNs thereof and transmit the reorderedpieces of uplink data. This solves the problem of an occurrence of dataloss in the SeNB.

In this case, though an amount of buffers necessary for the SeNBincreases compared with the method described above, the SeNB cantransmit the buffered uplink data to the T-MeNB immediately after achange of the MeNB configuration between the SeNB and the T-MeNB isperformed. This allows the uplink data from the UE to be delivered tothe T-MeNB early.

A specific example in which the UE transmits uplink data to the SeNBbefore notifying the T-MeNB of the completion of connection will bedescribed below.

After receiving MCI from the S-MeNB, the UE transmits new uplink data tothe T-MeNB via the SeNB. The T-MeNB notifies the UE of a headercompression configuration via the S-MeNB, and the UE uses the headercompression configuration in new uplink transmission with the T-MeNB viathe SeNB.

In this case, the UE may stop transmitting uplink data to the S-MeNB,which has been performed between the UE and the SeNB before receivingMCI. The UE may also transmit, to the SeNB, the uplink data whosetransmission to the S-MeNB has failed in the header compressionconfiguration configured by the T-MeNB. The UE may start transmittingdata to the T-MeNB via the SeNB, starting from the data with thesmallest PDCP SN among the pieces of data whose transmissions havefailed.

Alternatively, the UE may transmit both of the uplink data directed tothe S-MeNB and the uplink data directed to the T-MeNB to the SeNB. TheSeNB may determine which header compression is used to determine towhich MeNB the uplink data is directed. Or, in another method, theinformation indicating to which MeNB the uplink data is directed may beincluded in or added to the uplink data from the UE to the SeNB fortransmission. For example, this information is included as one bit. Inthe method of configuring one bit, for example, when the SeNB transmitsand receives only the user plane data, the D/C bit of a PDCP format maybe configured as a bit for the information.

The SeNB buffers the uplink data from the UE until a change of the MeNBconfiguration with the T-MeNB completes and, after the change of theMeNB configuration with the T-MeNB completes, transmits the uplink datafrom the UE to the T-MeNB. The SeNB may reorder the pieces of uplinkdata using the PDCP SNs thereof and transmit the reordered pieces ofuplink data.

The uplink data that the S-MeNB has received from the UE will bedescribed. As to the pieces of uplink data from the UE which have beenreceived by the S-MeNB, the uplink data received directly from the UE aswell as the uplink data received via the SeNB, may be managed inaccordance with the PDCP SNs. The S-MeNB reorders the pieces of uplinkdata in accordance with the PDCP SNs thereof and then transmits thereordered pieces of uplink data to the S-GW.

When the S-MeNB transmits MCI to the UE, the uplink data beingcommunicated directly between the UE and the S-MeNB in the transmissionof MCI may be transmitted from the S-MeNB to the S-GW after the data issuccessfully delivered to the S-MeNB. Similarly, the uplink data beingcommunicated between the UE and the S-MeNB via the SeNB in thetransmission of MCI may be transmitted to the S-GW after the data issuccessfully delivered to the S-MeNB. The S-GW may manage the order ofthe data received from the S-MeNB and the data received from the T-MeNB.

If the data whose transmission is determined to have completed at thetime when the UE receives the MCI is yet to be successfully delivered tothe S-MeNB for some reason, a data loss, which occurs because the UEdoes not transmit the data to the T-MeNB, can be prevented. The data maybe forwarded to the T-MeNB in place of being transmitted from the S-MeNBto the S-GW. The T-MeNB may reorder pieces of data in accordance withthe PDCP SNs thereof and transmit the reordered pieces of data to theS-GW. This achieves effects similar to those of the method oftransmitting data to the S-GW.

Whether the S-MeNB transmits the data to the S-GW or forwards the datato the T-MeNB may be configured. The entity that performs theconfiguration may be a node at the RAN side or the core network side.The configuration information may be notified to the S-MeNB in advance.For example, the MME or the S-GW notifies the S-MeNB of theconfiguration information in advance.

When the S-MeNB transmits MCI to the UE, the data successfully deliveredimmediately after the data whose transmission has failed may betransmitted to the S-GW or forwarded to the T-MeNB. The UE performsuplink transmission to the T-MeNB, starting from the data whosetransmission has failed. When the UE fails in delivering the relevantdata and the following pieces of data to the T-MeNB after the completionof HO, the forwarded data is available.

Disclosed below is a method of handling downlink data notified to the UEby the MeNB via the SeNB.

The S-MeNB that has received the handover request Ack message in stepST1203 stops transmitting new downlink data and stores the downlink datain the buffer. The downlink data may be all pieces of downlink datatransmitted to the UE that is caused to perform HO. The S-MeNB may stopthe transmission before the division into downlink data to betransmitted through a direct path to the UE and downlink data to betransmitted through a path to the UE via the SeNB, and store thesepieces of downlink data in the buffer. Alternatively, if the downlinkdata to be transmitted through the direct path to the UE and thedownlink data to be transmitted through the path to the UE via the SeNBare divided within the S-MeNB, the S-MeNB may stop transmitting thesepieces of downlink data and store them in the buffer. Numbering of PDCPSNs may be performed as the process for downlink data by the S-MeNB.

The S-MeNB forwards, to the T-MeNB, the downlink data that has not beendelivered and the following pieces of downlink data among the pieces ofdownlink data transmitted and received to and from the UE beforereceiving the handover request Ack message, or, before data transmissionis stopped.

The S-MeNB may forward, to the T-MeNB, pieces of downlink data that havenot been delivered, starting from the data with the smallest PDCP SN.The S-MeNB may forward, to the T-MeNB, pieces of downlink datatransmitted through the direct path from the S-MeNB to the UE as well aspieces of downlink data transmitted through the path via the SeNB,starting from the data with the smallest PDCP SN.

The S-MeNB starts forwarding to the T-MeNB in step ST1208, performs SNstatus transfer in step ST1209, and performs data forwarding in stepST1210. Forwarding is managed in accordance with the PDCP SNs, andforwarding is performed up to the end marker from the S-GW notified insteps ST942 and ST944.

In step ST1211, the T-MeNB stores the data forwarded from the S-MeNB inthe buffer.

This prevents an occurrence of downlink data loss in the HO process evenwhen the UE has performed dual connectivity using the SeNB.

The T-MeNB stores downlink data in the buffer until the direct RRCconnection reconfiguration with the UE completes and until the processof changing a MeNB by the SeNB completes. The T-MeNB may buffer thedownlink data through the direct path to the UE and the downlink datathrough the path via the SeNB before or after the separation by bearersplit. Both of these pieces of downlink data can be managed inaccordance with the PDCP SNs numbered. The UE can reorder the pieces ofdownlink data in accordance with the PDCP SNs.

In the method of storing pieces of downlink data in the buffer beforeseparation, in the case where the bearer split configuration using theSeNB is to be changed, scheduling for data separation suitable for newbearer split configurations is enabled after the new bearer splitconfigurations are configured for both of the paths. This enablesscheduling efficient for the UE during dual connectivity.

In the method of storing pieces of downlink data in the buffer after theseparation, when both of the paths become available, these paths can beused immediately to transmit downlink data to the UE. In other words, anincrease in delay amount can be prevented.

Upon completion of the direct connection with the UE and upon completionof the process of changing a MeNB by the SeNB, the T-MeNB performs splitscheduling of the pieces of downlink data to each path and transmits thedownlink data to the UE through each path.

Another method will be disclosed. The T-MeNB starts a downlink datatransmission to the UE upon completion of the direct connection with theUE or upon completion of the notification of a change of the MeNB by theSeNB, whichever comes earlier.

When storing pieces of data in the buffer before separating them, theT-MeNB performs downlink data transmission through an earlier path untilboth of the paths are established. After that, the T-MeNB may startbearer split when both of the paths are established.

When storing pieces of data in the buffer after separating them, theT-MeNB may perform downlink data transmission upon the establishment ofeach path.

This allows downlink data to be transmitted to the UE as early aspossible.

The SeNB is notified of the configuration of a bearer for dualconnectivity to the UE by the MeNB. The configuration informationrelated to RRC of the SeNB to the UE is notified by the MeNB.

The SeNB thus needs to recognize, for example, by which MeNB the SeNBhas been notified of the configuration of the bearer split, and to whichMeNB the SeNB should notify of the configuration information related toRRC that has been configured by the SeNB. In other words, the SeNB needsto recognize the MeNB.

The SeNB performs data communication for the UE that performs dualconnectivity between MeNBs. The SeNB needs to recognize the data fromwhich MeNB the SeNB should transmit to the UE, and to which MeNB theSeNB should transmit the data from the UE. In other words, the SeNBneeds to recognize the MeNB.

Although the former MeNB and the latter MeNB may be configuredseparately, the case where they are identical to each other will now bedescribed. This MeNB is referred to as a control MeNB for the SeNB.

Disclosed below is a method of recognizing, in the HO process by the UEduring dual connectivity, to which MeNB the control MeNB for the SeNBhas been changed in the case where the SeNB is not changed and the MeNBis changed from the S-MeNB to the T-MeNB. The following two, (1) and(2), will be disclosed as specific examples of the method of recognizingto which MeNB the control MeNB for the SeNB has been changed.

(1) The S-MeNB notifies the SeNB of a change of the control MeNB.

(2) The T-MeNB notifies the SeNB of a change of the control MeNB.

The specific example (1) will be disclosed below specifically. Uponreceipt of an RRC connection reconfiguration complete message from theUE, the T-MeNB notifies the S-MeNB of a message requesting to change thecontrol MeNB for the SeNB. The notification may be made through X2signaling. The identifier of a SeNB that is to change the control MeNBmay be included in this notification to be notified. The change causeinformation may be included to be notified. Or, the informationindicating that a change of the MeNB is due to HO may be provided to benotified.

The S-MeNB that has received this notification notifies the SeNB that isto change the control MeNB of a change of the control MeNB. Thisnotification may be made through X2 signaling. Alternatively, thisnotification may be made through signaling on an interface providedbetween the MeNB and the SeNB.

The following seven, (1) to (7), will be disclosed as specific examplesof the information included in the signaling.

(1) Information indicating a change of the control MeNB.

(2) Identifier of the control MeNB after change, which is herein theidentifier of the T-MeNB.

(3) Bearer identifier of a path using a SeNB, which is herein an E-RABidentifier (which may be, for example, an E-RAB ID) or an EPS beareridentifier.

(4) Identifier of a split bearer whose control MeNB is to be changed inthe SeNB.

(5) Identifier of a MeNB that has requested to configure a bearer splitconfiguration, which is herein the identifier of an S-MeNB.

(6) Identifier of a UE that performs dual connectivity using bearersplit.

(7) Combination of (1) to (6) above.

The SeNB that has received the control MeNB change notification messagespecifies a split bearer whose MeNB is to be changed from theinformation included in the change notification message. The SeNBchanges the control MeNB of the split bearer to a MeNB after change. TheSeNB may manage the split bearer in association with the identifier of acontrol MeNB. In other words, the SeNB may associate the identifier ofthe split bearer and the identifier of the control MeNB with each other.This allows the SeNB to change a control MeNB of the split bearer for aUE that performs dual connectivity.

After changing a control MeNB, the SeNB accepts a request to modify andrelease the split bearer only from the control MeNB after change.Additionally, the SeNB performs data communication with the MeNB afterchange.

This allows the SeNB to recognize to which MeNB the control MeNB hasbeen changed and to perform, between the control MeNB after change anditself, communication for control of, for example, a request to modifyand release the split bearer to the UE and data communication.

The SeNB that has changed the control MeNB may notify the S-MeNB of acontrol MeNB change response message. This message may include theinformation indicating that the control MeNB has been changed.

The S-MeNB that has received the control MeNB change response messagefrom the SeNB may notify the control MeNB after change, here, the T-MeNBof a message indicating that a change of the control MeNB for the SeNBhas completed.

This allows the MeNB after change, here, the T-MeNB to recognize thatthe control MeNB for the SeNB has been changed and to start, to theSeNB, communication for control of, for example, a request to modify andrelease the split bearer to the UE and data communication.

The MeNB after change may notify the SeNB of a message confirming thatthe control MeNB of the split bearer for the UE has been changed. TheSeNB may respond to the message.

The specific example (2) will be disclosed specifically. Upon receipt ofthe RRC connection reconfiguration complete message from the UE, theT-MeNB notifies the SeNB of a control MeNB change request message. Usedas the identifier of the SeNB may be the SeNB information included inthe HO request message received from the S-MeNB. This notification maybe made through X2 signaling or through signaling on an interfaceprovided between the MeNB and the SeNB. The information disclosed in themethod of the specific example (1) is applied as the informationincluded in the signaling.

The process by the SeNB that has received the control MeNB changerequest message from the T-MeNB is similar to that of the specificexample (1), and thus, description thereof will be omitted.

This allows the SeNB to recognize to which MeNB the control MeNB hasbeen changed and to perform, between the control MeNB after change anditself, communication for control of, for example, a request to modifyand release the split bearer to the UE and data communication.

The SeNB that has changed the control MeNB may notify the T-MeNB of acontrol MeNB change response message. This message may include theinformation indicating that the control MeNB has been changed.

The T-MeNB that has received the control MeNB change response messagefrom the SeNB may notify the control MeNB before change, here, theS-MeNB of a message indicating that the control MeNB for the SeNB hasbeen changed.

This allows the MeNB, here, the S-MeNB to recognize that the controlMeNB for the SeNB has been changed and to end the communication forcontrol of, for example, a request to modify and release the splitbearer to the UE and data communication to the SeNB.

When the UE during dual connectivity using the SeNB has performed HO,the SeNB can perform communication for control related to the splitbearer configuration and data communication with the MeNB after change(T-MeNB).

The UE and the T-MeNB can thus perform data communication via the SeNB.

A specific example of the process of changing a MeNB for the SeNB willbe described. In step ST1214, the T-MeNB that has received the RRCconnection reconfiguration complete message from the UE in step ST1213of FIG. 21 notifies the S-MeNB of a message requesting to change thecontrol MeNB for the SeNB. The identifier of the SeNB that is to changethe control MeNB is included in this message.

In step ST1215, the S-MeNB that has received this message notifies theSeNB that is to change the control MeNB of a message requesting tochange the control MeNB. This message includes the informationindicating a change of the control MeNB, the identifier of the T-MeNB,the bearer identifier of the path using the SeNB, the identifier of asplit bearer whose control MeNB is to be changed in the SeNB, theidentifier of the S-MeNB, and the identifier of the UE being a HO targetthat performs dual connectivity using the SeNB. This allows the SeNB torecognize for which split bearer configured by which MeNB the SeNB needsto change the control MeNB.

In step ST1216, the SeNB that has received the message uses theinformation received in step ST1215 to change the control MeNB. Thisallows the SeNB to perform communication for control related to thesplit bearer and data communication with the T-MeNB being a MeNB afterchange. The SeNB that has changed the control MeNB ends thecommunication for control and data communication with the control MeNBbefore change.

In step ST1217, the SeNB that has changed the control MeNB notifies theS-MeNB of a control MeNB change response message to notify that the SeNBhas changed the control MeNB. In step ST1218, the S-MeNB that hasreceived the response message notifies the T-MeNB of a messageresponding to the change of the control MeNB for the SeNB in order tonotify that a change of the control MeNB for the SeNB has completed.This allows the T-MeNB to recognize that the MeNB for the SeNB has beenchanged to the T-MeNB. The T-MeNB thus can perform communication forcontrol related to the split bearer and data communication with theSeNB.

Thus, in steps ST1220 and ST1221, the downlink data from the T-MeNB tothe UE and the uplink data from the UE to the T-MeNB can be communicatedvia the SeNB.

Steps ST1214 to ST1218 shown in FIG. 21 are referred to as step ST1222.Step ST1222 indicates the process of changing the MeNB for the SeNB thatis performed among the T-MeNB, the S-MeNB, and the SeNB.

The path switch process by the S-GW for changing the MeNB from stepST939 to step ST947 is similar to that of FIG. 12, and thus, descriptionthereof will be omitted.

Thus, data communication via the T-MeNB or via the T-MeNB and the SeNBcan be performed between the S-GW and the UE being a HO target.

As to the data communication between the S-GW and the UE, through onepath, data is directly communicated between the T-MeNB and the UE instep ST1005, and through the other path, data is communicated betweenthe T-MeNB and the UE via the SeNB in steps ST1007 and ST1008. In stepST1006, data is communicated through one path between the T-MeNB and theS-GW.

The method disclosed in this embodiment allows the UE during dualconnectivity by bearer split to perform HO between MeNBs.

Another method of the process of changing the MeNB for the SeNB in stepST1222 will be disclosed. In the method disclosed above, the process ofchanging the MeNB for the SeNB is performed after the T-MeNB receives anRRC connection reconfiguration complete message from the UE.

In the other method, the process of changing the MeNB for the SeNB maybe performed after the T-MeNB notifies the S-MeNB of a handover requestAck message, after the S-MeNB receives the handover request Ack messagefrom the T-MeNB, or after the S-MeNB transmits an RRC connectionreconfiguration message including MCI to the UE. The method describedabove is applicable to the process of changing the MeNB for the SeNB.

This allows the SeNB to change the control MeNB to the T-MeNB early, sothat data communication can be performed between the SeNB and the T-MeNBearly.

The UE stops transmitting uplink data until the RRC connectionreconfiguration with the T-MeNB completes and needs not to buffer uplinkdata. Or, the SeNB stops transmitting uplink data and needs not tobuffer the uplink data. The UE can communicate uplink data to the T-MeNBvia the SeNB early by split bearer using the SeNB.

The T-MeNB stops transmitting downlink data until it receives RRCconnection reconfiguration complete from the UE and needs not to bufferthe downlink data. The T-MeNB can communicate downlink data to the UEvia the SeNB early by split bearer using the SeNB.

Consequently, communication can be performed through the path via theSeNB between the UE and the T-MeNB even when data communication is yetto be performed through the direct path between the UE and the T-MeNB.This enables the data communication process during HO with low delay,thus reducing data loss during HO.

Another method of the process of changing the MeNB for the SeNB in stepST1222 will be disclosed. The process of changing the MeNB for the SeNBmay be performed after the T-MeNB completes the path switch process forthe MME and the S-GW. The T-MeNB notifies the SeNB of a change of theMeNB upon receipt of a path switch request Ack message from the MIVIE.

Alternatively, the S-MeNB may notify the SeNB of a change of the MeNBupon receipt of a UE context release message from the T-MeNB. The methoddescribed above may be applied to the process of changing a MeNB for theSeNB.

Thus, data communication by split bearer using the SeNB can be performedafter the HO process, including a path switch from the MME and the S-GWto the T-MeNB, completes. Data communication is performed after thereliable completion of the HO process, thus preventing control of datacommunication from becoming more complicated.

Fourth Embodiment

FIGS. 23 to 25 show an example sequence of a handover-related process ina communication system of a fourth embodiment of the present invention.FIG. 23 is continuous with FIG. 24 at a boundary BLS. FIG. 24 iscontinuous with FIG. 25 at a boundary BL6. The handover-related processof this embodiment is similar to the handover-related processes of thefirst embodiment shown in FIGS. 10 to 13, the second embodiment shown inFIGS. 17 to 19, and the third embodiment shown in FIGS. 20 to 22described above, and thus, the same steps will be denoted by the samestep numbers, and description thereof will be omitted.

In this embodiment, the method of handling downlink data differs fromthat of the third embodiment. This embodiment will disclose the methodof handling downlink data to be notified to the UE by the MeNB via theSeNB.

In this embodiment, the S-MeNB that has received a handover request Ackmessage in step ST1203 stops transmitting new downlink data and storesthe data in the buffer. The downlink data may be all pieces of downlinkdata transmitted to the UE that is caused to perform HO. The S-MeNB maystop the transmission after the division into downlink data to betransmitted through the direct path to the UE and downlink data to betransmitted through the path to the UE via the SeNB, store the pieces ofdownlink data in the buffer, and continue transmitting the downlink datato be transmitted through the path to the UE via the SeNB without astop. As the process for downlink data by the S-MeNB, PDCP SNs may benumbered.

The S-MeNB forwards, to the T-MeNB, the downlink data that has not beendelivered and the following pieces of downlink data among the pieces ofdownlink data transmitted and received to and from the UE beforereceiving the handover request Ack message, or, before stopping datatransmission. The S-MeNB may forward, to the T-MeNB, the pieces ofdownlink data that have not been delivered starting from the data withthe smallest PDCP SN. The S-MeNB may forward, to the T-MeNB, the piecesof downlink data transmitted from the S-MeNB through the direct path tothe UE starting from the data with the smallest PDCP SN.

The S-MeNB starts forwarding to the T-MeNB in step ST1301, performs SNstatus transfer in step ST1209, and performs data forwarding in stepST1210. The forwarding is managed in accordance with PDCP SNs, andforwarding is performed up to the end marker from the S-GW notified insteps ST942 and ST944.

The following steps are executed as in the third embodiment.Specifically, in step ST1211, the T-MeNB stores the data forwarded fromthe S-MeNB in the buffer.

This prevents an occurrence of downlink data loss during the HO processeven when the UE has performed dual connectivity using the SeNB.

In this embodiment, the T-MeNB stores downlink data in the buffer untila direct RRC connection reconfiguration with the UE completes and untilthe process of changing the MeNB by the SeNB completes. The T-MeNB maystore pieces of downlink data in the buffer after the separation intodownlink data through the direct path with the UE and downlink datathrough the path via the SeNB by bearer split. Pieces of downlink datacan be managed in accordance with PDCP SNs numbered. The UE can reorderthe pieces of downlink data in accordance with PDCP SNs.

In the method of storing downlink data in the buffer before separation,in the case where the bearer split configuration using the SeNB is to bechanged, after a new bearer split configuration is configured for bothof the paths, scheduling for data separation suitable for the bearersplit configuration is enabled. Thus, scheduling efficient for the UEduring dual connectivity can be performed.

In the method of storing downlink data in the buffer after theseparation, when both paths become available, downlink data can beimmediately transmitted to the UE using both of the paths. In otherwords, an increase in delay amount can be prevented.

The following steps are executed as in the third embodiment. Uponcompletion of the direct connection with the UE and upon completion ofthe process of changing a MeNB by the SeNB completes, the T-MeNBperforms split scheduling of pieces of downlink data to each path, anduses each path to transmit the downlink data to the UE.

Another method will be disclosed. The T-MeNB starts a downlink datatransmission to the UE upon completion of the direct connection with theUE or upon completion of the notification of a change of the MeNB withthe SeNB, whichever comes earlier.

Thus, downlink data can be transmitted to the UE as early as possible.

Fifth Embodiment

FIGS. 26 to 28 show an example sequence of a handover-related process ina communication system of a fifth embodiment of the present invention.FIG. 26 is continuous with FIG. 27 at a boundary BL7. FIG. 27 iscontinuous with FIG. 28 at a boundary BL8.

In this embodiment, before handover from the S-MeNB to the T-MeNB isperformed, both of the SeNB and the S-MeNB transmit and receive data toand from the UE using a bearer 2 (hereinafter also referred to as an“EPS bearer #2”) as a bearer corresponding to EPS (hereinafter alsoreferred to as an “EPS bearer”). During the handover process, only theSeNB transmits and receives data to and from the UE. After the handoverprocess, both of the SeNB and the T-MeNB transmit and receive data toand from the UE.

A UE that transmits and receives data to and from both of the SeNB andthe S-MeNB using the EPS bearer #2 will be described as an example. Whenthe UE performs handover from the S-MeNB to the T-MeNB, immediatelybefore handover, the EPS bearer #2 is shifted completely to route onlyvia the RLC/MAC/PHY of the SeNB without via the RLC/MAC of the S-MeNB.Alternatively, the EPS bearer #2 is shifted completely to route only viathe RLC/MAC/PHY of the SeNB without via the S-MeNB at layers lower thanthe S-GW, which include GTPu/PDCP. Alternatively, in place of beingshifted completely so as to route only via the RLC/MAC/PHY of the SeNB,the EPS bearers #2 is shifted completely so as to route via all of theGTPu/PDCP/RLC/MAC/PHY of the SeNB. In the description below, that thebearer is shifted completely so as to route via the SeNB may be referredto as that “the bearer is shifted completely to the SeNB”.

In step ST2001 of FIG. 26, the S-MeNB, the T-MeNB, the MME, and the S-GWprovide an area restriction (area restriction provided).

Step ST2002 to the HO decision of step ST2011 are performed as in thefourth embodiment described above. Specifically, in step ST2002, theS-MeNB notifies the UE of a measurement control message. Measurement ofa neighbor SeNB may be configured in the measurement control message.Measurement of a frequency for SeNB may be configured. Or, as ameasurement configuration, an event for SeNB or an event at a frequencyfor SeNB, or event criteria may be configured separately from that forthe MeNB.

Examples of the configuration parameter include a SeNB identifier, afrequency, an event number for report, a threshold for receptionquality, and a measurement period. Examples of the reception qualityinclude reference signal received power (RSRP) and reference signalreceived quality (RSRQ).

The UE that has received the measurement control message in step ST2002measures neighbor cells, that is, the MeNB and the SeNB.

In step ST2003, packet data is communicated between the UE and theS-MeNB. In step ST2004, packet data is communicated between the S-MeNBand the S-GW.

When the EPS bearer #2 is subjected to bearer split, packet data isdirectly communicated between the UE and the S-MeNB in step ST2005, andpacket data is directly communicated between the S-MeNB and the S-GW instep ST2006.

In steps ST2007 and ST2008, packet data is communicated between the UEand the S-MeNB via the SeNB. Specifically, packet data is communicatedbetween the UE and the SeNB in step ST2007, and packet data iscommunicated between the SeNB and the S-MeNB in step ST2008.

In step ST2009, the S-MeNB notifies the UE of UL allocation information.In step ST2010, the UE notifies the S-MeNB of a measurement reportmessage.

In step ST2011, the S-MeNB that has received the measurement reportmessage in step ST2010 uses the result of the measurement report todecide whether to cause the UE to perform handover (HO) to the T-MeNB.In the example shown in FIG. 26, in step ST2011, the S-MeNB decides tocause the UE to perform HO to the T-MeNB.

When deciding to cause the UE to perform HO to the T-MeNB in stepST2011, the S-MeNB moves to step ST2012. In step ST2012, the S-MeNBdecides whether to perform a modification to completely shift the EPSbearer #2 to the SeNB (hereinafter also referred to as a “modificationto completely shift the EPS bearer #2 to the SeNB”). In the exampleshown in FIG. 26, the S-MeNB decides to perform a modification tocompletely shift the EPS bearer #2 to the SeNB.

In step ST2013, the S-MeNB performs an EPS bearer #2 completeshift/no-shift confirmation process to the SeNB. Here, the EPS bearer #2complete shift/no-shift confirmation process to the SeNB refers to theprocess of confirming with the SeNB about whether the EPS bearer #2 canbe shifted completely to the SeNB, that is, whether a complete shift ofthe EPS bearer #2 to the SeNB can be made. Specifically, as the EPSbearer #2 complete shift/no-shift confirmation process, the S-MeNBnotifies the SeNB of a complete shift/no-shift confirmation signal bywhich whether the EPS bearer #2 can be shifted completely to the SeNB isconfirmed. The SeNB that has received the complete shift/no-shiftconfirmation signal notifies the S-MeNB of whether the EPS bearer #2 canbe shifted completely to the SeNB.

In step ST2014, the S-MeNB determines whether a complete shift to theSeNB can be made based on whether a complete shift can be made, whichhas been notified from the SeNB. The S-MeNB moves to step ST2015 ifjudging that a complete shift to the SeNB can be made or moves to stepST2016 if judging that a complete shift to the SeNB cannot be made.

In step ST2015, the UE, the SeNB, and the S-MeNB perform a process ofmodifying a complete shift of the EPS bearer #2 to the S-MeNB.Specifically, in step ST2015, the process of completely shifting the EPSbearer #2 to the SeNB is performed.

In step ST2016, the S-MeNB releases the SeNB and then executes the HOprocess as in step ST1102 of FIG. 17. In another embodiment of thepresent invention, if judging that a complete shift to the SeNB cannotbe made and moving to step ST2016, the S-MeNB may execute the HO processwithout releasing the SeNB.

In step ST2017, packet data is communicated between the S-MeNB and theS-GW. In step ST2018, packet data is communicated between the UE and theSeNB. In step ST2019, packet data is communicated between the SeNB andthe S-MeNB.

In step ST2020, the S-MeNB notifies the T-MeNB that is a HO destinationof a handover request message. The HO request message includes the SeNBinformation and the information about the EPS bearer #2 (hereinafter,also referred to as “EPS bearer #2 information”). The SeNB informationand the EPS bearer #2 information may be notified in a message differentfrom the HO request message.

In step ST2021, the T-MeNB determines whether the SeNB needs to bechanged. The T-MeNB moves to step ST2022 if judging that the SeNB needsto be changed or moves to step ST2023 if judging that the SeNB needs notto be changed.

In step ST2022, the T-MeNB releases the SeNB and then executes thehandover (HO) process as in step ST1102 of FIG. 17.

In step ST2023, the T-MeNB determines whether the bearer configurationneeds to be changed. The T-MeNB moves to step ST2025 of FIG. 28 ifjudging that the bearer configuration needs to be changed or moves tostep ST2024 if judging that the bearer configuration needs not to bechanged.

In step ST2024, the SeNB and the T-MeNB perform a MeNB changeconfirmation process to the SeNB. Here, the MeNB change confirmationprocess refers to the process of confirming whether to change a MeNBfrom the S-MeNB to the T-MeNB. Specifically, as the MeNB changeconfirmation process, the T-MeNB notifies the SeNB of a MeNB changeconfirmation signal by which whether to change a MeNB from the S-MeNB tothe T-MeNB is confirmed. The SeNB that has received the MeNB changeconfirmation signal notifies the T-MeNB of whether to change a MeNB. TheS-MeNB is not involved in the process of step ST2024.

In step ST2025 of FIG. 28, the UE, the SeNB, the S-MeNB, the T-MeNB, theMME, and the S-GW perform a MeNB HO process for EPS bearer #1. Also withthe NAS signaling via the SeNB, the MeNB HO process for EPS bearer #1 ofstep ST2025 is followed. The MeNB HO process for EPS bearer #1 will bedescribed below in detail.

In step ST2026, the SeNB, the S-MeNB, and the T-MeNB perform a MeNBchange process to the SeNB. This is the process of notifying that thecontrol plane (C-plane) for the SeNB, for example, the macro eNB (MeNB)that is to perform signaling has been changed after the completion ofthe handover of the MeNB. This process is a process necessary fornotifying that the MeNB of the EPS bearer #2 has been changed to theT-MeNB for data transmission and reception between the T-MeNB and theSeNB. Specifically, in step ST2026, the S-MeNB notifies the SeNB of asignal indicating that the MeNB has been changed to the T-MeNB.

In step ST2027, packet data is communicated between the UE and the SeNB.In step ST2028, packet data may be communicated between the SeNB and theT-MeNB.

In step ST2029, the T-MeNB determines whether to modify the EPS bearer#2 for a complete shift of the EPS bearer #2 to the SeNB. In the exampleshown in FIG. 28, the T-MeNB decides to modify the EPS bearer #2 for acomplete shift of the EPS bearer #2 to the SeNB.

As a result, the EPS bearer #2 is changed for transmission and receptionto and from only the SeNB not to be subjected to handover during theMeNB handover process, and thus, is unaffected by the handover. Thehandover process can thus be simplified, reducing handover failures.This also reduces dropped data.

In step ST2030, the UE, the SeNB, and the T-MeNB perform an EPS bearer#2 complete shift/no-shift confirmation process to the SeNB and amodification process to completely shift the EPS bearer #2 to the SeNB.The S-MeNB is not involved in the process of step ST2030.

In step ST2031, packet data is communicated between the UE and theT-MeNB. In step ST2032, packet data is communicated between the T-MeNBand the S-GW.

In step ST2033, packet data is communicated between the UE and the SeNB.In step ST2034, packet data is communicated between the SeNB and theT-MeNB.

FIGS. 29 and 30 show an example sequence of the MeNB HO process for EPSbearer #1 in step ST2025 of FIG. 28.

In step ST2041, the T-MeNB performs admission control to confirmaccommodation capacity as in step ST930 of FIG. 12 described above. Whenjudging that it can accept HO based on the result of the admissioncontrol, in step ST2042, the T-MeNB notifies the S-MeNB of a handoverrequest Ack message as in step ST931.

In response to the handover request Ack message of step ST2042, in stepST2043, the SeNB, the S-MeNB, and the T-MeNB perform a MeNB changeprocess to the SeNB. Here, the MeNB change process is a process ofnotifying that the control plane (C-plane) for the SeNB, for example,the macro eNB (MeNB) that performs signaling has been changed in thedata flows from the T-MeNB to the S-MeNB and further from the S-MeNB tothe SeNB, or in the data flows from the T-MeNB to the S-MeNB and fromthe T-MeNB to the SeNB. Specifically, the S-MeNB notifies the SeNB of asignal indicating that the MeNB has been changed to the T-MeNB. That theMeNB has been changed may be notified in a handover request Ack messageof step ST2042.

In step ST2044, packet data is communicated between the UE and theS-MeNB. In step ST2045, packet data is communicated between the S-MeNBand the T-MeNB.

In step ST2046, the S-MeNB notifies the UE of DL allocation information.In step ST2047, the S-MeNB notifies the UE of an RRC connectionreconfiguration message including mobility control information. In stepST2047, the S-MeNB may notify that there are no changes in the SeNB andthe EPS bearer #2.

The processes from steps ST2048 to ST2055 are similar to those of 3GPPTS36.300. Specifically, in step ST2048, the UE detaches itself from theS-MeNB that is an old cell and starts synchronization with the T-MeNBthat is a new cell.

In step ST2049, the S-MeNB transfers, to the T-MeNB that is a targeteNB, the packet being stored in the buffer and the packet beingtransmitted.

In step ST2050, the S-MeNB performs SN status transfer to transfer thestatus of the sequence number (SN) of the PDCP to the T-MeNB, as in stepST912 of FIG. 11. In step ST2051, the S-MeNB may perform data forwardingto forward yet-to-be-transmitted data to the T-MeNB.

In step ST2052, the T-MeNB stores the packets transferred from theS-MeNB in the buffer.

In step ST2053, the UE is synchronized with the T-MeNB. In step ST2054,the T-MeNB notifies the UE of UL allocation information and a trackingarea (TA) for the UE. In step ST2055, the UE notifies the T-MeNB of anRRC connection reconfiguration complete message.

In step ST2056 of FIG. 30, the SeNB, the S-MeNB, and the T-MeNB performthe process of changing a MeNB to the SeNB as in step ST2026 of FIG. 28.This is the process of notifying that the control plane (C-plane) forthe SeNB, for example, macro eNB (MeNB) that performs signaling has beenchanged.

In step ST2057, packet data may be communicated between the UE and theT-MeNB. In step ST2058, packet data is communicated between the UE andthe SeNB. In step ST2059, packet data may be communicated between theSeNB and the T-MeNB. In step ST2060, the T-MeNB may transfer packet datato the S-GW.

In step ST2671, the T-MeNB, the MME, and the S-GW perform a path switchrequest to request a change of both of the paths of the EPS bearer #1and the EPS bearer #2 from the S-MeNB to the T-MeNB.

Specifically, in step ST2061, the T-MeNB notifies the MME of a pathswitch request message. In step ST2062, the MME that has been notifiedof the path switch request message notifies the S-GW of a modify bearerrequest message.

In step ST2063, the S-GW that has been notified of the modify bearerrequest message changes a downlink path. In step ST2064, the S-GW mayprovide an end marker to the PDCP to be transmitted to the S-MeNB,thereby informing about the end of the forwarding process. In stepST2066, the S-MeNB may provide an end marker and forward it to theT-MeNB. In step ST2065, packet data may be communicated between theT-MeNB and the S-GW.

In step ST2067, the S-GW notifies the MME of a modify bearer responsemessage. In step ST2068, the MME that has been notified of the modifybearer response message notifies the T-MeNB of a path switch request Ackmessage indicating the completion of path switch. The process of stepST2671 completes in this way.

In step ST2069, the T-MeNB notifies the S-MeNB of a UE context releasemessage. In step ST2670, the S-MeNB that has been notified of the UEcontext release message releases the resource allocated to the UE. Afterthe resource release process of step ST2670, the MeNB change process tothe SeNB in step ST2026 of FIG. 28 is performed.

As described above, in the process of step ST2029 of FIG. 28, only theSeNB is configured to transmit and receive data to and from the UE usingthe EPS bearer #2 during handover. How to perform the process of stepST2029 will be described with reference to FIG. 31.

FIG. 31 shows an example status of the data transmission and receptionto and from the UE. An S-GW 601 includes a PDCP processing eNB switchunit 602. An S-MeNB 603 includes a first PDCP processing unit 604, anRLC processing unit 605, a MAC processing unit 606, a PHY processingunit 607, and a second PDCP processing unit 608. A SeNB 609 includes aPDCP processing unit 610, a PDCP path switch unit 611, an RLC processingunit 612, a MAC processing unit 613, and a PHY processing unit 614.

Before handover switching, a UE 615 transmits and receives data to andfrom both of the S-MeNB 603 and the SeNB 609 using the EPS bearer #2.For example, for downlink, the S-GW 601 provides data to the first PDCPprocessing unit 604 and the second PDCP processing unit 608 of theS-MeNB 603. The first and second PDCP processing units (hereinafter,collectively referred to as “PDCP processing units” as well) 604 and 608perform the PDCP process in LTE or LTE-A.

The data provided to the second PDCP processing unit 608 is provided tothe PDCP path switch unit 611 of the SeNB 609. The PDCP path switch unit611 switches the path of the PDCP. The PDCP path switch unit 611 is notduring handover, and thus, determines to provide the PDCP from thesecond PDCP processing unit 608 to the RLC processing unit 612 andprovides the data from the second PDCP processing unit 608 to the RLCprocessing unit 612. The RLC processing unit 612 performs the RLCprocess in LTE or LTE-A.

The data provided to the RLC processing unit 612 is then sequentiallyprovided to the MAC processing unit 613 and the PHY processing unit 614,and is subsequently provided to the UE 615 through radio transmission.The MAC processing unit 613 performs the MAC process in LTE or LTE-A.The PHY processing unit 614 performs the PHY process in LTE or LTE-A.

When only handover of the MeNB is performed with the data transmissionand reception in SeNB 609 unchanged as in this embodiment, datatransmission and reception using the EPS bearer #2 are not performedbetween the UE 615 and the S-MeNB 603 during handover.

When shifting completely to the SeNB 609 including the PDCP, the S-GW601 causes the PDCP processing eNB switch unit 602 to switch an eNB thatprocesses the PDCP. The PDCP processing eNB switch unit 602 transmitsdata using the EPS bearer #2 not to the second PDCP processing unit 608of the S-MeNB 603 but to the PDCP processing unit 610 of the SeNB 609.The PDCP processing unit 610 performs the PDCP process in LTE or LTE-A.

The SeNB 609 that has received the data from the S-GW 601 performs thePDCP process on the received data by the PDCP processing unit 610, andprovides the data after the process to the PDCP path switch unit 611.The SeNB 609 selects not the PDCP data from the second PDCP processingunit 608 of the S-MeNB 603 but the PDCP data from the PDCP processingunit 610 of its own device by the PDCP path switch unit 611, and thenprovides the selected data to the RLC processing unit 612.

The PDCP data provided to the RLC processing unit 612 is then providedto and processed by the MAC processing unit 613 and the PHY processingunit 614 in the stated order, and is eventually transmitted to the UE615 through radio transmission.

The provision of the PDCP processing eNB switch unit 602 and the PDCPpath switch unit 611 enables a process of transmitting data by only theSeNB that is not to be subjected to handover during handover of theMeNB.

The process flow in uplink is as in downlink. Also in uplink, as indownlink, the provision of the PDCP processing eNB switch unit 602 andthe PDCP path switch unit 611 enables a process of transmitting dataonly by the SeNB that is not to be subjected to handover during handoverof the MeNB.

The process, performed when the SeNB and the T-MeNB perform datatransmission and reception using the EPS bearer #2 after the handoverprocess in step ST2025 of FIG. 28, is executed as follows. Duringhandover, the SeNB changes its connection from the S-MeNB to the T-MeNB.After that, the T-MeNB changes the path for transmitting and receivingdata only to and from the SeNB to the paths for transmitting andreceiving data to and from the SeNB and the T-MeNB. In a conceivablealternative method, connection is performed through the path fortransmitting and receiving data of both of the SeNB and the T-MeNBduring handover.

On that occasion, a radio resource configuration is conceivably changedin two ways: (A) by configuring the configuration of a split bearer fromthe T-MeNB, and (B) by changing a switch of a data transmission andreception path.

The following four, (A-1) to (A-4), will be disclosed as specificexamples of the way in which a radio resource configuration is changed(A) by configuring the configuration of a split bearer from the T-MeNB.

(A-1) The information that needs to be notified to the T-MeNB by theS-MeNB is the SeNB identification information, specifically, theinformation about an address of the SeNB. This is because at the timethe data subjected to the PDCP process is transmitted to the SeNB, theSeNB identification information is necessary. In the absence of the SeNBidentification information, the T-MeNB does not know to which SeNB itshould transmit data, and from which SeNB it should receive data.Additionally, the T-MeNB needs to know parameter information indicatingin accordance with which RRC connection parameter the SeNB operates, andthus, this information is also notified.

(A-2) The information that needs to be notified to the S-MeNB by theT-MeNB is the information indicating whether handover of a MeNB hassucceeded or failed. When the data stays in the buffer of the S-MeNB,the information indicating a command to perform data forwarding is alsothe information that needs to be notified to the S-MeNB by the T-MeNB.

(A-3) The information that needs to be notified to the UE is anotification indicating that the MeNB has been switched by handover fromthe S-MeNB to the T-MeNB. In this case, the UE transmits and receivesdata not to and from the S-MeNB but to and from the T-MeNB.

(A-4) In the configuration of the split bearer, the SeNB transmits andreceives data to and from the S-GW via the PDCP processing unit of theS-MeNB or the T-MeNB. The information that needs to be notified to theSeNB is the information indicating the PDCP processing unit of the MeNB,that is, the S-MeNB or the T-MeNB, via which the SeNB is routed.

The following four, (B-1) to (B-4), will be disclosed as specificexamples of the way in which a radio resource configuration is changed(B) by changing a switch of a data transmission and reception path.

(B-1) The information that needs to be notified to the T-MeNB by theS-MeNB is the SeNB identification information. This is because if theS-MeNB does not know with which SeNB it has performed simultaneouscommunication before handover, it does not know with which SeNB itshould perform simultaneous communication after handover. Additionally,the T-MeNB needs to know parameter information indicating in accordancewith which RRC connection parameter the SeNB operates, and thus, theT-MeNB is also notified of that information. Assumed in the specificexample (B-1) is a case where the SeNB handles only data transmissionand reception and the T-MeNB handles control information (signaling).

(B-2) The information that needs to be notified to the S-MeNB by theT-MeNB is, as in the specific example (A-2), the information indicatingwhether handover of the MeNB has succeeded or failed and thenotification instructing to perform data forwarding when the data staysin the buffer of the S-MeNB.

(B-3) The information that needs to be notified to the UE is, as in thespecific example (A-3), the notification indicating that the MeNB hasbeen switched by handover from the S-MeNB to the T-MeNB. In this case,the UE transmits and receives data not to and from the S-MeNB but to andfrom the T-MeNB.

(B-4) The information that needs to be notified to the SeNB is theinformation indicating that the MeNB has been switched from the S-MeNBto the T-MeNB upon completion (success) of handover. This is because theSeNB depends on the MeNBs and thus needs to know a MeNB, on which theSeNB depends, transmitting and receiving a control signal to and fromits own device, for example, signaling from among the MeNBs and tofollow the control from the relevant MeNB.

In the sequence shown in FIGS. 26 to 30, a radio resource configurationis changed at thee timings: (1) step ST2026, (2) step ST2056, and (3)step ST2043. Each of the timings (1) to (3) above will be specificallydescribed below.

(1) At the timing of the MeNB change process to the SeNB in step ST2026,a radio resource configuration is changed after the timing of changing adownlink path in step ST2063. Thus, the MeNB has been switched to theT-MeNB and the resource of the S-MeNB has been released, therebypreventing an erroneous process by the resource before handover.Additionally, a radio resource configuration can be changed in a simpleprocess, and the process can be enabled by a relatively small circuit.

(2) At the timing of the MeNB change process to the SeNB in step ST2056,a radio resource configuration is changed before the timing of changinga downlink path in step ST2063. This results in a contradiction inwhich, before the higher-layer entity is switched from the S-MeNB to theT-MeNB by handover, the radio communication with the UE has become theoperation that should be performed after handover radio.

This leads to a complicated process compared with the case where a radioresource configuration is changed at the timing (1). However, thanks toan early switch timing, a risk that data transmission and reception areperformed only by the SeNB, for example, a situation in which access iscentered on the SeNB from a plurality of UEs and a congestion occurs canbe accordingly avoided, enabling a stable operation.

(3) The timing of the MeNB change process to the SeNB in step ST2043 isimmediately after the T-MeNB notifies of an Ack response to the handoverrequest in step ST2042. At this timing, a radio resource configurationis changed at a timing earlier than the timing (2).

Although this results in, for example, a more complicated processperformed when handover fails than in the case where a radio resourceconfiguration is changed at the timing (2), the above-mentioned risk canbe avoided more reliably, thus enabling a more stable operation.

It is only the uplink data that can be transmitted and received with theconfiguration of a radio resource at the timings (2) and (3). Downlinkdata is transmitted and received after the T-MeNB receives data from theS-GW.

Sixth Embodiment

FIGS. 32 and 33 show an example sequence of a MeNB HO process for EPSbearer #1 in a communication system of a sixth embodiment of the presentinvention. FIG. 32 is continuous with FIG. 33 at a boundary BL10.

While the S-MeNB performs a reconfiguration in the fifth embodimentdescribed above, the T-MeNB performs a reconfiguration in thisembodiment. Specifically, in this embodiment, the processes similar tothose of the fifth embodiment are performed except for that a MeNB HOprocess for EPS bearer #1 in step ST2680 shown in FIGS. 32 and 33 isperformed in place of step ST2025 shown in FIGS. 29 and 30 in thesequence of the fifth embodiment shown in FIGS. 26 to 30 describedabove.

The process of step ST2680 shown in FIGS. 32 and 33 is similar to theMeNB HO process for EPS bearer #1 of step ST2025 shown in FIGS. 29 and30, and thus, the same steps will be denoted by the same step numbers,and common description will be omitted.

The process of step ST2680 is similar to the process of step ST2025 ofthe fifth embodiment except for that in step ST2025 of the fifthembodiment shown in FIGS. 29 and 30, the processes of steps ST2681 andST2682 are performed in place of steps ST2046 and ST2047 of FIG. 29.

In this embodiment, in step ST2681, the T-MeNB notifies the UE of DLallocation information. In step ST2682, the T-MeNB also notifies the UEof an RRC connection reconfiguration message. In this embodiment, theRRC connection reconfiguration message does not include mobility controlinformation.

Although the S-MeNB notifies the UE of an RRC connection reconfigurationmessage in the fifth embodiment described above, the T-MeNB notifies theUE of an RRC connection reconfiguration message in this embodiment.

This allows not the S-MeNB being a handover source but the T-MeNB beinga handover destination, which is to perform communication connection, toperform an optimal RRC connection configuration, thus stabilizing thecommunication between the UE and the MeNB.

Seventh Embodiment

FIGS. 34 and 35 show an example sequence of a MeNB HO process for EPSbearer #1 in a communication system of a seventh embodiment of thepresent invention. FIG. 34 is continuous with FIG. 35 at a boundaryBL11.

In this embodiment, processes similar to those of the fifth embodimentare performed except for that a MeNB HO process for EPS bearer #1 ofstep ST2690 shown in FIGS. 34 and 35 is performed in place of stepST2025 shown in FIGS. 29 and 30 in the sequence of the fifth embodimentshown in FIGS. 26 to 30 described above.

The process of step ST2690 shown in FIGS. 34 and 35 is similar to theMeNB HO process for EPS bearer #1 of step ST2025 shown in FIGS. 29 and30, and thus, the same steps will be denoted by the same step numbers,and common description will be omitted.

The process of step ST2690 is similar to the process of step ST2025 inthe fifth embodiment except for that the process of step ST2691 isperformed in addition to the process of step ST2025 of the fifthembodiment shown in FIGS. 29 and 30.

In this embodiment, before steps ST2046 and ST2047, in step ST2691, theT-MeNB notifies the S-MeNB of an RRC connection reconfiguration messageand DL allocation information.

In this embodiment, as described above, the T-MeNB notifies the S-MeNBof the reconfiguration message, and then, the S-MeNB notifies the UE ofthe contents of the reconfiguration message. Consequently, the S-MeNBdoes not notify the UE of the reconfiguration message as in the sixthembodiment described above, but notifies the UE, via the S-MeNB, of thecontents of a reconfiguration message notified by the T-MeNB.

As in the sixth embodiment, consequently, an optimal RRC connectionconfiguration is performed not by the S-MeNB being a handover source butby the T-MeNB being a handover destination, which is to performcommunication connection, thus stabilizing the communication between theUE and the MeNB.

The embodiments and the modifications thereof are also applicable to thecase where a SeNB configures a plurality of serving cells. Similarly,they are also applicable to the case where a MeNB configures a pluralityof serving cells. The cluster of serving cells configured by a SeNB maybe referred to as a secondary cell group (SCG), and a cluster of servingcells may be referred to as a master cell group (MCG).

The embodiments and the modifications thereof are merely an illustrationof the present invention and can be freely combined within the scope ofthe invention. Also, the elements of the embodiments and themodifications thereof can be appropriately modified or omitted.

While the invention has been shown and described in detail, theforegoing description is in all aspects illustrative and notrestrictive. It is therefore understood that numerous modifications andvariations can be devised without departing from the scope of theinvention.

DESCRIPTION OF REFERENCE NUMERALS

51 S-MeNB, 52 coverage of S-MeNB, 53 T-MeNB, 54 coverage of T-MeNB, 55and 58 SeNB, 56 and 59 coverage of SeNB, 57 UE.

1. (canceled)
 2. A communication system comprising: a communicationterminal; and at least one base station that configures a plurality ofcells for performing radio communication with the communicationterminal, wherein the plurality of cells include: a first cell; a secondcell that provides a first dual connectivity along with the first cell;and a third cell that provides a second dual connectivity along with thefirst cell, and the communication terminal is configured to keep aconnection with the first cell while the communication terminal performsa handover from the second cell to the third cell for shifting from thefirst dual connectivity to the second dual connectivity.
 3. Thecommunication system according to claim 2, wherein a configuration of abearer that has been configured using the first cell is not changedwhile the communication terminal performs the handover from the secondcell to the third cell.
 4. A communication terminal that is used in acommunication system comprising the communication terminal, and at leastone base station that configures a plurality of cells for performingradio communication with the communication terminal, wherein theplurality of cells include: a first cell; a second cell that provides afirst dual connectivity along with the first cell; and a third cell thatprovides a second dual connectivity along with the first cell, and thecommunication terminal is configured to keep a connection with the firstcell while the communication terminal performs a handover from thesecond cell to the third cell for shifting from the first dualconnectivity to the second dual connectivity.
 5. A base station that isused in a communication system comprising a communication terminal, andat least one base station that configures a plurality of cells forperforming radio communication with the communication terminal, whereinthe plurality of cells include: a first cell; a second cell thatprovides a first dual connectivity along with the first cell; and athird cell that provides a second dual connectivity along with the firstcell, the first cell and the second cell are configured by the basestation or another base station, and the third cell is configured by thebase station, and the base station is configured to: determine whether aconfiguration of a bearer that has been configured using the first cellcan be handed over from the second cell in a case where thecommunication terminal performs a handover from the second cell to thethird cell for shifting from the first dual connectivity to the seconddual connectivity; and decide to keep a connection between thecommunication terminal and the first cell during the handover whendetermining the configuration of the bearer can be handed over from thesecond cell.